power: handle WakeUpScreen from gnome-shell

This signal should be handled in the same way as resetting
IDLETIME, but we don't really reset the counter, so other
parts of the code (for example the automatic suspend) are
unaffected.

https://bugzilla.gnome.org/show_bug.cgi?id=726056
power: don't unblank the screen for notifications

The Shell already does that.

https://bugzilla.gnome.org/show_bug.cgi?id=726056