gst/realmedia/: Added RDT depayloader.

gst/realmedia/: Added RDT depayloader.

Original commit message from CVS:
* gst/realmedia/Makefile.am:
* gst/realmedia/rdtdepay.c: (gst_rdt_depay_base_init),
(gst_rdt_depay_class_init), (gst_rdt_depay_init),
(gst_rdt_depay_setcaps), (gst_rdt_depay_chain),
(gst_rdt_depay_set_property), (gst_rdt_depay_get_property),
(gst_rdt_depay_change_state), (gst_rdt_depay_plugin_init):
* gst/realmedia/rdtdepay.h:
Added RDT depayloader.
* gst/realmedia/rmdemux.c: (gst_rmdemux_parse_mdpr),
(gst_rmdemux_plugin_init), (plugin_init):
Remove unused function.
Added some more mime-types.