desktop-shell: Avoid spurious configure events for xdg-shell activation

This is a minor re-work of how we de-activate and activate the surfaces
in desktop-shell. As activate() is being used for handling keyboard
input events, this basically rectifies that such that  activation
happens only if the passed surface is different from the currently
focused surface.

Fixes: #593

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