shell: Use swapped signal connection for MetaWindowActor::destroy

The callback would schedule a clutter_actor_destroy() on the first argument. Unless told otherwise, this is the same actor that is already being destroyed, so this is a no-op.

Instead, the intent is to make the clone follow up destruction of the window actor so that it results in correct removal of this window from the ShellWindowPreviewLayout. Use a swapped connection to pass the clone actor as the first argument of the signal callback.

Fixes: 04c78167 ("Move WindowPreviewLayout from JS to C")

Closes: #6570 (closed)