window-base: Use g_object_new instead of clutter_stage_new
window-base: Use g_object_new instead of clutter_stage_new

Create the object explicitly instead of using the convenience function.
This is necessary to bypass the multiple-stage check on single-window
backends, as Clutter will always create the default stage.

Work-around suggested by the Clutter maintainer, Emmanuele Bassi.

jonnylamb/mx,pwithnall/mx,pwithnall/mx,clutter-project/mx,jonnylamb/mx,pwithnall/mx,jonnylamb/mx,jonnylamb/mx,clutter-project/mx,clutter-project/mx,pwithnall/mx,clutter-project/mx