Add extra functions for allowing bindable hotkeys from applets, desklets, extensions.

Added:

meta_display_rebuild_keybindings
meta_display_add_xlet_keybinding
meta_display_remove_xlet_keybinding

to allow runtime keybinding assignments without gsettings involvement.

This is a dependency of https://github.com/linuxmint/Cinnamon/pull/1803