wayland: Use correct window resource when sending parent_window

While not common a client can have bound the global multiple times
and create window objects through all of them. In this case the event
send to the client would carry the wrong object since only the clients
of the two resources were checked for equality.
We can save the resource of the window managment the window
belongs to by extending the resource and storing it there.
Sending the initialState has been moved out of resource_bind since
it happens directly after allocate() and there being no opportunity
to set the custom wmResource field in between.