Gnome 3.12

I've updated gnome to 3.12. I'm probably going to merge tomorrow if you don't see any problem at a first glance.
Noteworthy changes:

 1. Moved gnome-3/* to gnome-3/3.10, and created gnome-3/3.12
 1. New scope `pkgs.gnome3_12`.
 1. New packages (not updates): cogl_1_18, clutter_1_18, upower_99, appdata-tools, spidermonkey_24
 1. Updates that should not break: geoclue 2.0.0 -> 2.1.8, json-glib 0.16.0 -> 1.0.0
 1. New `environment.gnome3.packageSet` nixos option for specifying which gnome3 to use, e.g. `pkgs.gnome3_12`.
 1. We are enforcing python3Packages = python33Packages due to #2693 and in general because python 3.3 is more stable for now.
 1. Disabled gnome-terminal runtime migration from gconf to dconf, to drop the only gconf dependency.