the "bubble" phase used to run before event handlers before GTK_PHASE_TARGET
was added, in order to keep phases in the expected order, move GTK_PHASE_BUBBLE
to be run (still invariably) after event handlers.

The only behavioral change should be wrt widgets wanting mixed event handler/
gesture handling, they could previously attach the gesture to the bubble phase
and check for gtk_gesture_is_active() in the event handler to bail out, they'll
have to use GTK_PHASE_CAPTURE for that purpose from now on.