Add plugin to generate a pattern detectable by videodetect.

Add plugin to generate a pattern detectable by videodetect.

Original commit message from CVS:
* docs/plugins/Makefile.am:
* docs/plugins/gst-plugins-bad-plugins-docs.sgml:
* docs/plugins/gst-plugins-bad-plugins-sections.txt:
* docs/plugins/inspect/plugin-videosignal.xml:
* gst/videosignal/Makefile.am:
* gst/videosignal/gstvideomark.c: (gst_video_mark_set_caps),
(gst_video_mark_draw_box), (gst_video_mark_420),
(gst_video_mark_transform_ip), (gst_video_mark_set_property),
(gst_video_mark_get_property), (gst_video_mark_base_init),
(gst_video_mark_class_init), (gst_video_mark_init),
(gst_video_mark_get_type):
* gst/videosignal/gstvideomark.h:
* gst/videosignal/gstvideosignal.c: (plugin_init):
Add plugin to generate a pattern detectable by videodetect.