API: Let multiple backends register the Windowed Output API

The windowed output API is currently implemented by the Wayland, the X11 and the headless backends. It isn't possible to create a secondary headless backend when the primary backend is Wayland or X11 because the windowed output API would be registered twice. This commit prefixes the windowed output API names with the backend name in order to avoid clashes: weston_<backend>_windowed_output_api_v2.