Fix the check that keeps events on overlay widgets from entering the
tool event mechanism, they have no business there.

gimp_overlay_child_realize(): set the embedding widget's event mask on
all overlay children, so their windows will be used as event window,
so their events become distinguishable from events on the parent (the
canvas).

gimp_display_shell_canvas_tool_events(): fix the check for events on
overlays, and skip them for real this time.