sys/: Call gst_x_overlay_prepare_xwindow_id() to give applications a final chance to set their own xwindow id before ...

Original commit message from CVS:
* sys/ximage/ximagesink.c: (gst_ximagesink_setcaps):
* sys/xvimage/xvimagesink.c: (gst_xvimagesink_setcaps):
Call gst_x_overlay_prepare_xwindow_id() to give applications
a final chance to set their own xwindow id before the video
sink creates its own window.