ges: Use a `ges:` uri to define timeline from description

ges: Use a `ges:` uri to define timeline from description

This way the command line formatter actually uses an URI and not
an ugly hack where were passing a random string instead of an URI.
This also allows the `gessrc` element to handle timelines described
in its URI meaning that you can now use, for example:

   gst-play-1.0 "ges:+test-clip blue d=4.0

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-editing-services/-/merge_requests/227>