Update GnomeRRScreen

Based on gnome-desktop commit: 2c7a2e90fee98ff61e530098bc962a65dec670bc
From: Giovanni Campagna <gcampagna@src.gnome.org>

- store priv as a local variable
- chain up in finalize (this was actually a bug)
- make :gdk-screen readwrite
- add a ::changed vmethod for the signal
- remove the callback in gnome_rr_screen_new, and drop mate_rr_screen_destroy
Make the signal be GnomeRRScreen::changed, not screen-changed, to match the vmethod name

Based on gnome-desktop commit: cce5c87297ff8c163d4a29e1fc4e7277cbb4705a
From: Federico Mena Quintero <federico@gnome.org>