Draft: qt: make dummy compositor wayland aware (naive wayland integration)

This is a naive approach for supporting display servers which support alpha compositing in non top-level windows.

It works for Wayland QPA, and possibly Windows QPA (It's TODO).

This can not work on X11, because it apparently only supports alpha in top-level windows (if at all).

Unfortunately, for some reason, key events are not respected by the inner Qt Quick View. I will take a look at that later, if this is accepted as an idea.

Resizing when video is playing is also cumbersome, but I guess it is not related to the changes here.