Disable mousekeys with Numlock ON
GNOME documentation on accessibility features states that mousekeys work only when NumLock is OFF:
https://help.gnome.org/users/gnome-help/stable/mouse-mousekeys.html
Change the implementation in mutter to match the documentation, i.e. disable mousekeys when NumLock in ON so that switching NumLock ON restores the numeric keypad behaviour.
Closes: #530 (closed)