encodebin: Add a profile-string property.

Submitted by Mathieu Duponchelle @meh

Link to original bug (#751469)

Description

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
serialization.

This will allow encodebin to be used from the command-line,
and allow removing code in devtools and GES.