WIP: thunderbolt: New toggle for autoenrollment

gnome-shell#884

Depends on gsettings-desktop-schemas!11 (closed) and gnome-shell!334 (closed).

When the screen is unlocked, it's convenient to automatically enroll new
Thunderbolt devices. However, you may want to disable this for
additional security (eg, to avoid being fooled by lookalike devices).

This doesn't affect automatic authorization.

Maybe something like this?:

This MR implements the above image. But I feel like it might look better in the style below (with the white background box and the clear rows):

If you think this MR is useful but want it to look more like the second image (or something else entirely!), I can have a look at making that happen ☺