Draft: Add globalshortcuts editing
This is a rough draft, and I'm looking for feedback on the high-level shape of it.
Most important is that I added another type of key, which exists to have its own gsettings interface, and to notify the compositor whenever the value changes.
The part where shortcuts are added is completely missing :) I have it as part of gnome-shell but I don't know how to turn it into standalone gjs (imports fail).