agl-shell-desktop: Add the ability to hide client windows

agl-shell-desktop: Add the ability to hide client windows

This would be particularly useful to pop-up kind of window(s).

It implements the 'deactivate_app' request. For the pop-up role we
just remove the view from the layer while for the desktop we store
the previous surface and re-use it if that's set. Otherwise we just
display the background surface.

As we now allow to hide client windows, we need to able to activate/show
them back so this also adds a slight modification to take care to handle
the pop-up role separately when calling the 'activate_app' request.

Bug-AGL: SPEC-3269

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: Iec5ccbe2815f4b0e32086fd49856f5f338147f79