prefs: Update overlay-key on settings changes

When changing the overlay-key setting, the change only takes effect
on restart - there are actually two bugs involved:
  (1) the test whether the key has changed is located in the
      else part of a test for string settings (and overlay-key happens
      to be a string settings ...)
  (2) with (1) fixed, a change signal is emitted, which triggers a
      reload of all keybindings - unfortunately, the actual value
      of overlay-key is only read on startup, so the key is reloaded
      using the old value
Fix both issues by replacing the custom handling of the overlay-key
with the regular handling of string preferences.

https://bugzilla.gnome.org/show_bug.cgi?id=681906