This is issue 869 and is required for several reasons: 1) to make Inishell more
similar to MeteoIO, Snowpack, etc 2) because Qt6 is moving to CMake in
replacement of qmake 3) because it will "easily" allow creating native packages
for several platforms.

Now the PreviewEdit is properly parented and the garbage collector can
cascade down to the syntax highlighters (which are parented through
their text document).

- INI syntax highlighting moved from PreviewWindow to PreviewEdit
- Split up BSD code with separate header because most of it is actually
our own by now...

Those shortcuts are system-dependent (and in fact don't even work on my
Ubuntu), so now we try to fetch the native key sequences.
Added a few more why not...

Now we have small icons to make things clearer

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

- The commanding lambda now correctly receives a constant index and
  therefore the correct tab name is changed.
- When previewing an INI file where all that has been done was to open
  an application without changing anything, it is now still marked as
  unsaved (because although it might save a click in special situations
  it does not make sense to have a different logic than in the GUI and
  can only be confusing).
- Default names now get an .ini file extension i. e. unsaved(1).ini.
- Path/filename splitting is done once on file changes and then reused
  from outside without asterisk shuffling.
- Minor logic cleanup, consts and styling

On the lon term, I would prefer Ctrl+Left / Right to change tabs
compared to the more complex Tab key shortcut. But this would require
setting an event filter on the QPlainTextEdit to re-assign the shortcut
(we don't necessarily need to move word by word here).

Files opened from the filesystem are now read only and not marked as
"modified". A tooltip listing the major shortcuts has been added. The
regex for coordinates has been improved (it is now the same as at the
other places in Inishell).

Otherwise, re-ordering of the variables declaration for better
alignment.

The drag & drop behaviour of the text editor itself is kept as-is
(moving text blocks with the mouse etc., and also inserting file names
if dropped into the text area) because we don't want to re-implement all
of this ourselves.
However, we display a guidance text if a user drags files over the text
area telling them where to drop them to open.

And some cosmetic fixes for the line numbering of the preview editor

Moreover, the license for the lines numbering has been clarified and is
now advertised in the About window

Modernize signal mapping in PreviewEdit (better compile time checks), re-add line number safety margin (sorry, my oversight).

(old style cast, and "no out-of-line virtual method definitions")

Use recommended call for text width. I looked this up a while ago for the Alternative width calculation. This way we stay happy with the default Qt 5.9 and don't require 5.11 just for this call.

(Ok I admit - I don't want to update Qt myself. But luckily it is the correct call.)

The fonts are now setup in the constructor. An unnecessary script has
been deleted.

And a minor visual tweak to the line numbering

(it is licensed under BSD instead of GPL)