... to prepare having "valid" icon

Now the user can choose to restart INIshell immediately if any of the
appearance settings have been modified when saving.

This way we can style the bg color (missing?) and font color (valid?)
without a clash on empty values.
Thus empty vals are now "valid", and if this is not wanted, it makes
sense to mark as non-optional.

A decision is needed how to manage (in)valid expressions.

If the default value of the panel was the same as the minimum value (as
per the XML) the Number would stay empty (because internally no signal
that the value has changed is emitted). Only 0 had been considered but
not other minimum values.
Quick fix: Style as non-empty whenever a change request is registered,
probably leading to some duplicate calls of this for "good" cases.

When both a default value and precision=... where provided in the xml
file, the precision was reset to the default precision. Now the default
precision is reset to the XML provided precision.

In order to give the user the required flexibility to enter the numeric
values he/she wants to, precision specifications have been almost
everywhere in the xml files (a few places still rely on the default
settings because it makes sense). The units have also been added where
possible.

The virtual stations are given as "VSTATIONx", not "POSITIONx"!
Moreover, more links to the online documentation have been added and the
syntax highlighting (in the PreviewWindow) now supports the coordinates
specifications (otherwise, coordinates provided with ";" were shown as
comments).

Now MeteoIO provides a first application: meteoio_timeseries and it is
handled by the meteoio workflow (instead of the hacky data_converter).

Small touch up on the Numbers: default decimal precision is now 2 (for a
precision up to the percent)

Implement mechanism to translate custom syntax to tinyexpr before styling (so e. g. hardcoded substitutions are allowed exactly like in some filters)

The static function felt like bad design ever since the Textfield
started using it...

Allow : and _ in INI key names for preview syntax highlighting (not perfect but better). Set tooltip for Number panel when an expression is used.

A small inconsistency has been fixed in Number.cc (now using the
background color instead of hard-coded white). The PATH is now expanded
to include the directory where inishell is (this will be convenient for
packaging with small applications). The NSIS templates for Windows have
been copied, but there is still work to do before it can be useful.

The oxygen PNG icons have been removed (they were not used anymore), new icons have been added and the globe and forumla icons replaced by svg icons. All icons are either self-made or from breeze/elementary.

Wrong (non-interactive) panels were passed to the INIParser. Not grave because the value
would be empty, but still this was fixed. In fact, start removing IDs
from non-interactive panels. They were given in the beginning so that
specific elements could be targeted, but this was never needed. So now
there's less object IDs flying around. There are probably more that will
be cleaned when stumbled upon, until then they do no harm.

Wrong (non-interactive) panels were passed to the INIParser. Not grave because the value
would be empty, but still this was fixed. In fact, start removing IDs
from non-interactive panels. They were given in the beginning so that
specific elements could be targeted, but this was never needed. So now
there's less object IDs flying around. There are probably more that will
be cleaned when stumbled upon, until then they do no harm.

'Alternative' panels. Better error messages.

Now for nodes where we know that the performance saving workaround
makes sense, we can explicitly set it in the attributes. By default,
all children are built. Result: E. g. GEO_HEAT will be filled correctly
even if it's not used because SNP_SOIL is FALSE (without having to state
that it should work in the XML).

Do mini TODO: start showing a hidden Number when the minus sign is entered. It's not perfect but better.

The active/inactive icons have been redone as svg. Formulas are now marked as invalid as long as they are...

More work on empty Number panels: starting with a key press now works as expected (more or less). Choose hiding color dependent on whether it's optional. Fix bug where clearing a Number via the context menu would not set the appropriate styling in motion. Feel free to revert to tiny font if more things clash...

Prevent help texts and the expression switch button from focussing on tab-clicks - can tab through settings much more smoothly

The tiny font for hiding a spinbox value makes the panel tiny. Maybe like this instead? (Use generic function name in case it changes over time)

Now, all keys (including those having a default value) are printed out. This is not ideal but until another solution is implemented, required in order for the generated ini files to be usable. Moreover, default values in spin hboxes are now written with a very small font in order to hide them from view... Again, not ideal but it will do until a better solution is implemented.

Replacing assignement operators by direct assignement in constructor and some constification

The color scheme has been redesigned and now follows Solarized (https://en.wikipedia.org/wiki/Solarized_(color_scheme)). Instead of directly coding colors in the code, they have all received symbolic names and then the names are mapped to a specific color. On the long term, we will be able to offer several color schemes, like Solarized, osX, Windows, etc

Add short info text about why the INIParser's == check asserts false, fix special case where an integer would be transformed to a float with decimals (thus failing the equality check)

Because there was no way to completely reset a Numbers panel (there can't not be a number in a spin box) this is now possible via a generic context menu in all panels. Restructure the menu a little to make these functions more clear

Add description of ZRXP plugin to MeteoIO XML. Add XML shortcut 'h' for 'help', minor styling. Iniqueryparser handles substitutions in keys now. Remove obsolete warning.