Previously we only trigger unblanking or showing the unlock dialog if
core key or motion events come through. Now we also do this when xinput
events come in. We also now reset the last-user-activity time for core
events.
This also changes xcb-xembed to explicitly only handle core events.
This includes an abortive attempt to do a gtk4 dialog (which I don't
think is possible, as gtk4 doesn't allow embedding toplevels anymore),
and an iced dialog, which I just never started writing.
