...and then make GWidget layout invalidation lazy. This way we coalesce
multiple invalidations into a single relayout and we don't have to worry
about child widgets not being fully constructed.
- child_at(Point)
- move_to_front()
- move_to_back()
- is_frontmost()
- is_backmost()
This patch also makes it possible to receive the mouse event that triggers
a context menu before the context menu is shown. I'm not sure this is the
best design for context menus but it works for now.
