Draft: keyboard: add a switch to enable/disable the switch-to-application* bindings

This adds a toggle for enabling or disabling the bindings <Super>1, <Super>2, etc. The switch enables or disables all of them at once. The switch is at the top of the keyboard settings, not in any section, due to how keyboard settings are configured in xml. This is similar to the alternate characters key option added in !19 (merged).

This is meant to resolve gnome-shell#1250 (closed). The other obvious solution to this, which is simpler, is to simply list all of these bindings separately, like any other binding. This makes it rather a pain to change them all. Perhaps worse, this would noticeably increase the vertical space used in the keyboard settings (the design being discussed at Teams/Design/settings-mockups#14 might help that somewhat.)

This is tagged as WIP because I still need to implement collision checking, both when using this toggle and when setting another binding that would conflict with this. (Implementing this could be awkward due to how collisions are currently handled.)

But I'm interested to see if anyone has input on this solution. Any thoughts?