tooltip: Look at the event's target widget

tooltip: Look at the event's target widget

Since we now have a widget whenever we query tooltips, we can as well
get the events target_widget if we have an event (which is what we do
when coming from gtkmain.c). This keeps us from searching the entire
widget hierarchy for the target event even though we've already done
that for pointing events in gtkmain.c

This reduced the work done in gtk_tooltip_handle_event in normal motion
events to basically nothing since we already did all the heavy lifting
when handling the pointing event in gtkmain.c