utils: Add a "gst_element_create_stream_id" method
Submitted by Edward Hervey
Currently the various gst_pad_create_stream_id functions requires a GstPad. But if you use the stream_id argument ... it doesn't use that pad.
For stream providers (like demuxers) who wish to create stream ids (based on upstream information) without creating pads it would be great to have similar functions that don't require a pad.