keybindings: Add external grab API

During compositor grabs, all global keybindings that don't go
through mutter's keybinding system are blocked. To allow other
processes to make use of it, gnome-shell will expose a simple
grab API on DBus; for this, add API to grab key combos directly
instead of parsing accelerators stored in GSettings.

https://bugzilla.gnome.org/show_bug.cgi?id=643111