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.

On recent version of MacOS, the dark theme is very well supported is now the
default. Unfortunately, our Breeze icons were suited for a light theme. So now
the dark version of Breeze has been added and the getIcons() function checks
which version of the icons should be used.

We need this shortcut for the tabs, and this way CMD+Q quits the whole
application as is expected on mac.
We can still use this shortcut by repeating it until all tabs are closed
(and ESC also works).

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

Add 'new tab' preview menu, listen to the print command (to show preview) in the preview itself, the Logger and the About window. Show the logger on STRG+L from the preview and about windows.

This had been done for the main window, but the About, Preview and Logger windows still had the problem. The About window is slightly wider (100 pixels more) and the text has been edited to be a little more compact (as well as showing the project's links closer to the top).

This icon set is to be used on Mac.