gst/multipart/multipartdemux.*: Don't blindly copy the mime-type as the caps name because they not always map directl...

Original commit message from CVS:
Patch by: Patrick Radizi <patrick dot radizi at axis dot com>
* gst/multipart/multipartdemux.c: (gst_multipart_demux_class_init),
(gst_multipart_demux_get_gstname),
(gst_multipart_find_pad_by_mime), (gst_multipart_demux_chain):
* gst/multipart/multipartdemux.h:
Don't blindly copy the mime-type as the caps name because they not
always map directly. Instead use a hashtable with common mappings.
Fixes #533287.