Avoid needless event copies when queueing from a backend to a stage

All backends follow the same pattern of queueing events first in
ClutterMainContext, then copying them to a ClutterStage queue and
immediately free them. Instead, we can just pass ownership of events
directly to ClutterStage thus avoiding the allocation and copy in
between.

https://bugzilla.gnome.org/show_bug.cgi?id=711857