compositor: introduce management of transientParent property

Signed-off-by: Christophe Chapuis <chris.chapuis@gmail.com>
compositor: don't create a window in surfaceMapped if there is already one existing (after hide/show for example)

Signed-off-by: Christophe Chapuis <chris.chapuis@gmail.com>
compositor: remove useless setPaintEnabled, already managed by the window

Signed-off-by: Christophe Chapuis <chris.chapuis@gmail.com>