remote-display: Merge plugin into XSettings

We merge the remote display object into XSettings, so that
we can override the enable-animations settings without modifying
GSettings. This was especially problematic on startup, or when
overriding user settings.

UNTESTED

https://bugzilla.gnome.org/show_bug.cgi?id=694692
media-keys: Merge power_action_* functions

They only differed by a single line.
media-keys: Make the "shutdown" action not be interactive

Shutdown is supposed to shutdown without asking, as "interactive"
already should do what we need for interactive shutdowns.

https://bugzilla.gnome.org/show_bug.cgi?id=698733