docs/plugins/: Add videocrop to docs.

docs/plugins/: Add videocrop to docs.

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/gst-plugins-bad-plugins.hierarchy:
Add videocrop to docs.
* gst/videocrop/Makefile.am:
* gst/videocrop/gstvideocrop.c:
* gst/videocrop/gstvideocrop.h:
Move boilerplate stuff and structures into a header file.
* tests/check/Makefile.am:
* tests/check/elements/.cvsignore:
* tests/check/elements/videocrop.c: (video_crop_get_test_caps),
(test_unit_sizes), (videocrop_test_cropping_init_context),
(videocrop_test_cropping_deinit_context),
(videocrop_test_cropping), (test_cropping), (videocrop_suite):
Add unit tests for videocrop.