gst/realmedia/: Add simple rdt manager.

gst/realmedia/: Add simple rdt manager.

Original commit message from CVS:
* gst/realmedia/Makefile.am:
* gst/realmedia/rdtmanager.c: (find_session_by_id),
(create_session), (free_session), (gst_rdt_manager_base_init),
(gst_rdt_manager_marshal_BOXED__UINT_UINT),
(gst_rdt_manager_class_init), (gst_rdt_manager_init),
(gst_rdt_manager_finalize), (gst_rdt_manager_query_src),
(gst_rdt_manager_chain_rtp), (gst_rdt_manager_chain_rtcp),
(gst_rdt_manager_set_property), (gst_rdt_manager_get_property),
(gst_rdt_manager_provide_clock), (gst_rdt_manager_change_state),
(create_recv_rtp), (create_recv_rtcp), (create_rtcp),
(gst_rdt_manager_request_new_pad), (gst_rdt_manager_release_pad),
(gst_rdt_manager_plugin_init):
* gst/realmedia/rdtmanager.h:
* gst/realmedia/rmdemux.c: (plugin_init):
Add simple rdt manager.