Wrap the existing laptop_display_is_on() method in a public function
that gnome-shell can use to query whether a builtin output is present
and enabled.
https://bugzilla.gnome.org/show_bug.cgi?id=765267
This just exposes the type and the singleton getter necessary to make
it available to introspection. We'll expose more functionality as it
becomes needed.
https://bugzilla.gnome.org/show_bug.cgi?id=743745