stage: Normalize key focus setting

We often mean that when key_focus == NULL, it's assumed to be on the
stage, and clutter_stage_get_key_focus() reflects this. We also do a
lot of check around the lines of key_focus == NULL instead of also
checking for the stage, so make sure to normalize it so that explicitly
grabbing the stage's key focus will not change our behaviour.

https://bugzilla.gnome.org/show_bug.cgi?id=683301