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.

... in addition to XML highlighting

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...

- 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

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.

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

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

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

(it is licensed under BSD instead of GPL)