libdeng2|Widget: Notification about removed children

The notification is now done after the removal has already occurred.
This is to allow observers to perform actions reflecting the parent's
state that no longer includes the child.
UI|Client|DialogWidget: Modal dialog flashes borders to indicate blocking

If an event gets eaten because a modal dialog is open, the dialog's
borders now briefly flash to let the user know that the dialog is
preventing the event from going further.