Don't disable ConsoleKit with new upower

ConsoleKit is still useful, even if we can't suspend/hibernate using
upower anymore.  Don't disable the whole thing.

https://bugzilla.gnome.org/show_bug.cgi?id=727099
NEWS: update for release
configure: bump to 3.12.1
autostart: Stop messing with XDG_CURRENT_DESKTOP

The use of the XDG_CURRENT_DESKTOP variable predates the actual
standardization that only happened recently, and indeed does not
work when the variable contains a colon-separated list as allowed
by the new standard. Stop using it altogether, so GIO can handle it
for us (which it will soon).

https://bugzilla.gnome.org/show_bug.cgi?id=729575