Sending a new PlasmaWindow as an event is quite problematic. The main
problem here is that it sends an id of 0 and this doesn't make sense
when the interface gets bound and all existing windows are send to the
client.

This changes the interaction to have the client create the resource for
the PlasmaWindow instead of the server creating it. The server generates
"window ids" and sends those to the client. The client now binds a
PlasmaWindow with the "window id" in the normal way.

In case the server doesn't know the id any more, it directly sends an
unmapped and destroyes the newly created resource again. It is not a
protocol error as that can happen (common example: file open dialog of
Qt 5 applications on Xwayland).