Only create decoration once during Client::manage

Client::manage calls setupCompositing on the not
yet fully managed client before it wants to create
the window decoration. This means the decoration is
created twice and maybe even if not needed (noBorder
not yet set).

If the Client is not yet managed setupCompositing
will not update the decoration - it will be handled
by manage later on.

Furthermore if compositing is not active, setup
compositing does not have to perform any action.

REVIEW: 104758