GstEncodingProfile should take multiple presets

Submitted by Arun Raghavan @arun

Link to original bug (#742831)

Description

The encoding API provided by GstEncodingProfile only allows for a single GstPreset to be attached to it. This means that we must know, before hand, what encoder will be used, which defeats the point of the encoding API.

Ideally, we should allow the provision of multiple presets to the GstEncodingProfile, probably with an associated GstElementFactory name. This would allow us to select the appropriate preset based on what is available on the system, and apply that.

Even better would be for the saved preset itself to have the associated factory name -- that way we would just walk down the list of provided presets and try to apply them till one works.

Relatedly, the GstEncoding*Profile should probably use properties for initialisation and getting/setting parameters.