kiosk-shell: Add a border fill to non-fullscreen views

According to xdg-shell spec, if the surface doesn't cover the whole
output we should center it and install a border fill covering the rest
of the output.

While we center out the surface we never got around installing the
border fill. This patch re-uses the activation of a surface to control
this bit as well, by making use of an new layer to place the
surface while not being active.

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