omxvideoenc: implement GstPreset interface
omxvideoenc: implement GstPreset interface

To allow user to use GstPreset to quickly save and load a set of
parameters.

https://bugzilla.gnome.org/show_bug.cgi?id=767907

CapOM/gst-omx,GStreamer/gst-omx,varunkumara/test-repo,varunkumara/test-repo,varunkumara/test-repo,GStreamer/gst-omx,CapOM/gst-omx,CapOM/gst-omx,GStreamer/gst-omx