[VoiCutter] solve keys, min and max intensity, and add waiting toolbox

[mathildemerle]

Fix https://github.com/Inria-Asclepios/music/issues/1063

This PR:

• Solve key problem on macOS.
• Min and max intensities are kept after output is displayed on view (@fcollot in pipeline it could be useful too, setting Window/Level directly seems to bug, but setting Min and Max intensity instead is ok).
• Add a waiting toolbox when computation is done.

By the way, there is a Tab command in Voi Cutter, but it's not clear how it's working.

:m: