Add Window::moveResizeOutput()

The Window::moveResizeOutput() property is used to track the output
where the window is expected to land after the move or resize operation
completes.

This can be used to decouple the current output from the next output,
which allows us to send better xdg_toplevel.configure_bounds events or
make windows stick to outputs while keeping Window::output() in sync
with the current output layout.