Update surface item position when it's created

Set surface item position according to the surface as soon as it's
created. Unlike toplevel surfaces, transient surfaces have coordinates
which are relative to the parent.
Updating the surface item position at creation time makes such surfaces
appear at the right position.

Change-Id: I33073b03c352e592ae9e0f54ddf417f93453869e
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Reviewed-by: Andrew Knight <andrew.knight@digia.com>