Disable naked Super keybinding if sticky

With sticky keys, users should be able to press and release a
modifier and then press a key to activate a modifier-key combination.
Activating the overview on the Super key release keeps these
users from using keyboard shortcuts involving the Super modifier.

The solution implemented here is to simply disable the Super-release
binding if sticky keys are enabled. It is still possible to go
to the overview by using Super-S or Alt-F1.
https://bugzilla.gnome.org/show_bug.cgi?id=685974