[macOS] enable shortcuts

[mathildemerle]

From an issue on Inria-Asclepios#169

On macOS the shortcuts in Qt applications are disabled by default: https://doc.qt.io/qt-6/qshortcut.html#details

This PR adds the methods which enable shortcuts.

I put the PR in draft since it needs to be tested. It can be tested with the Exit pop-up.

Ⓜ️

[Florent2305]

Interesting, just need to test it

[mathildemerle]

Tested on macOS:

• on master branch, the native macOS menu bar that we use have its own keyboard shortcut to access: control + fn + F2
• as said by Qt shortcut documentation on macOS even if we enable the shortcuts, the underlined letters as done on linux and windows are not shown in the native menu bar on macOS
• this qt setting in this PR could be used by other widgets, for instance the "Do you really want to exit?" popup. However, it's not working...

So, the qt shortcut system on macOS is not stable for now on Qt5. We can keep this PR to test when we'll be on Qt6. I put this PR as invalid for now.