Request paint if scene changes for the canvas item

Otherwise for every scene the canvas is simply not painted anymore in the
designer. For example if you change the parent.

Change-Id: I4883eab44d73a59381230755f7caa596f2b6569f
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>