sys/: Remove client-side overlay handling, use the X-server v4l plugin for that. Nicer overlay, less code. Also make ...

Original commit message from CVS:
* sys/v4l/Makefile.am:
* sys/v4l/gstv4l.c: (plugin_init):
* sys/v4l/gstv4lelement.c: (gst_v4lelement_get_type),
(gst_v4lelement_init), (gst_v4lelement_dispose),
(gst_v4lelement_change_state):
* sys/v4l/gstv4lelement.h:
* sys/v4l/gstv4lxoverlay.c: (gst_v4l_xoverlay_open),
(gst_v4l_xoverlay_close), (idle_refresh),
(gst_v4l_xoverlay_set_xwindow_id):
* sys/v4l/gstv4lxoverlay.h:
* sys/v4l/v4l-overlay_calls.c:
* sys/v4l/v4l_calls.h:
* sys/v4l2/Makefile.am:
* sys/v4l2/gstv4l2.c: (plugin_init):
* sys/v4l2/gstv4l2element.c: (gst_v4l2element_get_type),
(gst_v4l2element_init), (gst_v4l2element_dispose),
(gst_v4l2element_change_state):
* sys/v4l2/gstv4l2element.h:
* sys/v4l2/gstv4l2xoverlay.c: (gst_v4l2_xoverlay_open),
(gst_v4l2_xoverlay_close), (idle_refresh),
(gst_v4l2_xoverlay_set_xwindow_id):
* sys/v4l2/gstv4l2xoverlay.h:
* sys/v4l2/v4l2-overlay_calls.c:
* sys/v4l2/v4l2_calls.h:
Remove client-side overlay handling, use the X-server v4l plugin
for that. Nicer overlay, less code. Also make the plugin
compileable without X (but then without overlay, obviously).
Makes xwindowlistener obsolete, should we remove that?