window-actor: Don't show actor until meta_window_actor_show()

By default clutter will show an actor as it is added to a parent. This means that after we create the window actor, when it's added to the window group, we implicitly show it. What we really want is to not show it until the window is supposed to be shown, which happens when meta_window_actor_show() is called, as showing prior to that, could cause issues.

Avoid the implicit show by setting the "show-on-set-parent" property on the window actor to FALSE on window actor construction.

Fixes #572 (closed)