shell: Start implementing the popup surface type

shell: Start implementing the popup surface type

This lands the basic behavior of the popup surface type, but there are still
a number of details to be worked out.  Mainly there's a hardcoded timeout
to handle the case of releasing the popup button outside any of the
client windows, which triggers popup_end if it happens after the timeout.
Maybe we just need to add that as an argument, or we could add a new event
that fires in this case to let the client decide whether it ends the popup
or not.