compositor: Ignore some of the builtin keybindings

Mutter on wayland currently has a bug where it crashes if the run dialog keybinding is pressed. No one notices the bug for gnome-shell, since gnome-shell overrides mutters run dialog keybinding.

This commit makes GNOME Kiosk also override the keybinding to avoid the crash. At the same time it neuters a few other builtin keybindings that aren't so useful to GNOME Kiosk.