gst/playback/: Make caps explicitely available. Makes testing for unsupported types...

gst/playback/: Make caps explicitely available. Makes testing for unsupported types possible. Improves error reporting.

Original commit message from CVS:
* gst/playback/gstplaybasebin.c: (unknown_type),
(add_element_stream), (new_decoded_pad),
(gst_play_base_bin_change_state):
* gst/playback/gststreaminfo.c: (gst_stream_info_class_init),
(gst_stream_info_init), (gst_stream_info_new),
(gst_stream_info_dispose), (gst_stream_info_get_property):
* gst/playback/gststreaminfo.h:
Make caps explicitely available. Makes testing for unsupported
types possible. Improves error reporting.