QtWayland shell integration for layer shell

[plfiorini]

We currently use a QML library for layer shell on the client side that doesn't integrate very well with the platform plugin.
Windows are made visible as soon as they are configured and when they are hidden and shown again, they are not recreated.
Also, it's hard to handle popup and modal windows but with a shell integration plugin we could hide all those details very well.

The idea is to make a LayerSurface interface exposed to clients written with QML, then the shell integration plugin finds the interface associated with the window and gets all the layer surface attributes from it.
While the shell integration plugin depends on a private API and will need to be rebuilt every time Qt is updated, the interface can be public.