sys/: Use a mutex protected list to marshal navigation events into the stream thread from whichever thread sends them.

Original commit message from CVS:
* sys/ximage/ximagesink.c: (gst_ximagesink_chain),
(gst_ximagesink_send_pending_navigation),
(gst_ximagesink_navigation_send_event), (gst_ximagesink_finalize),
(gst_ximagesink_init):
* sys/ximage/ximagesink.h:
* sys/xvimage/xvimagesink.c: (gst_xvimagesink_chain),
(gst_xvimagesink_send_pending_navigation),
(gst_xvimagesink_navigation_send_event),
(gst_xvimagesink_finalize), (gst_xvimagesink_init):
* sys/xvimage/xvimagesink.h:
Use a mutex protected list to marshal navigation
events into the stream thread from whichever thread
sends them.