Add XWayland api to not decorate XWayland windows

To be able to use libweston as a basis for a compositor that does server-side window decorations (treating in the same way native Wayland windows and XWayland ones), it should be possible to suppress window decorations for XWayland windows. The proposed modification introduces the new api call xserver_no_window_decorations_loaded that acts as xserver_loaded but suppresses adding decorations and shadows to XWayland windows.