desktop-shell: Properly handle lowered fullscreen surfaces

desktop-shell: Properly handle lowered fullscreen surfaces

lower_fullscreen_surface() was removing fullscreen surfaces from
the fullscreen layer and inserting them in the normal workspace
layer. However, those fullscreen surfaces were never put back in
the fullscreen layer, causing bugs such as unrelated surfaces
being drawn between a fullscreen surface and its black view.

Change the lower_fullscreen_surface() logic so that it lowers
fullscreen surfaces to the workspace layer *and* hides the
black views. Make this reversible by re-configuring the lowered
fullscreen surface: when it is re-configured, the black view
will be shown again and the surface will be restacked in the
fullscreen layer.

https://bugs.freedesktop.org/show_bug.cgi?id=73575
https://bugs.freedesktop.org/show_bug.cgi?id=74221
https://bugs.freedesktop.org/show_bug.cgi?id=74222