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.

Now MacOS uses the flat-bw icons instead of elementary (whose icons are not always that understandable)

(and use a separate #define DEBUG_TIMERS for it)

I always expect something to happen on Strg+F in the preview... so the find function has been implemented.

Outource some code instead of tripling it. Closing the preview via key
board is a bit more intuitive (strg+w closes one tab after the other
until it finally closes the main window).

Add the same mechanism of inclusion/exclusion of sections for <options> (Dropdown, Choice, Grid, ...)