encoding-profile: Let the user decide what preset name to use

encoding-profile: Let the user decide what preset name to use

It was possible to decide only what #GstElement implementing #GstPreset
to use during the encoding, we can now let the user select a specific preset previously
saved using #gst_preset_save_preset specifying the name chosen when it was saved
in the gst_encoding_profile_set_preset_name.

Actually loading a preset with %NULL as a name would have always failed, so
in the current state of the API that feature is unusable

API:
  gst_encoding_profile_set_preset_name
  gst_encoding_profile_get_preset_name