This way, new higher numbers get put in the back.

... instead of unsorted at the end.

The QTimer now always uses the original stylesheet avoiding fetching one
that is already colored (and restoring to that).

When opening context help the topic can now be anywhere on the page.
Also: QTabWidget::clear() clears the tabs but does not delete
anything... oops. Now we do this manually potentially saving big amounts
of memory and lookups.

Now it works multiple times!

I don't think we will allow to preview the help contents...

Some further restructuring

This is an intermediate commit.

We need to be able to set the tab bar everything gets built into... so
first, split up some functionality.

Add base functionality to the new Help window.
For now, it displays all content in one window.

Getting the tabs right will take a couple of commits... (an additional
abstraction layer is needed so that the tab bar can be exchanged and is
not coupled to the main window)

Also duplicated headers plus better ordering

If an unknown section is encountered, check our list of dynamic sections
if it fits and should be created.

It is more of a Replicator than a Selector...

Since the tab index will change when removing other tabs, the lambda now
emits the tab name to look for and delete.

This implies having a clear parent Section with a "+", and the
duplicated templates can be removed with a "-".

Revert to idea of using a global container - it would be possible to do
it without and always search on the fly, but readability would suffer...

Now dedicated <section> tags may be flagged as templates, and the
respective tabs will show a custom button to take some action to be
implemented.

This is to be displayed next to the tab names for dynamic sections.

Merge fix for issue 868

We must suppress setting a property because in the Replicator this is to
request creating new template panels.

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

This is to check combinations of mandatory/default values for all
panels.

... and vice versa. I. e. if "optional = false" is set for either the
Selector or the Template panel, both are marked non-optional.
This way it is in both directions and analogous to the natural way the
Replicator behaves.

This commit also gets rid of the transferral of the "optional" property
between template widgets and the Selector.
While it may not make much sense to have a non-optional Selector with an
optional template, it avoids confusion if the properties can just be set
separately.

This way widgets that call the styling functions directly will also work
without modifications.