appIconMenu: Disable sourceActor key events
Enter/space are already taken over by the app icon to activate the app, and arrow keys interfere with keyboard navigation where icons are arranged in a grid (namely the app picker).
Also as app icons only create the menu when it is first needed, the shortcuts only starts to work once a menu has been opened, which is rather inconsistent and confusing.
It is better to simply disable the key handling altogether for the app icon menu.
Fixes #98