Add support for the layer-shell v1 protocol

The layer-shell protocol allows wayland clients create surfaces that can be used for desktop environment components such as panels, docks, notifications, on screen display windows, etc.

Once plasma in all its entirety is ported to the layer-shell protocol, support for our proprietary plasma-shell protocol will be dropped.

In order to test this patch, you need kwayland-server!67 (merged).