encodebin: Add a profile-string property.
Submitted by Mathieu Duponchelle
This code was written by Thibault Saunier, and used in
GES and gst-devtools.
The documentation is lifted from gst-devtools.
It is write-only for now, I don't yet see a use-case for
This will allow encodebin to be used from the command-line,
and allow removing code in devtools and GES.