mpegts: Unify API : GstMpegts / gst_mpegts_ / GST_*_MPEGTS_

mpegts: Unify API : GstMpegts / gst_mpegts_ / GST_*_MPEGTS_

It was previously a mix and match of both variants, introducing just too much
confusion.

The prefix are from now on:
* GstMpegts for structures and type names (and not GstMpegTs)
* gst_mpegts_ for functions (and not gst_mpeg_ts_)
* GST_MPEGTS_ for enums/flags (and not GST_MPEG_TS_)
* GST_TYPE_MPEGTS_ for types (and not GST_TYPE_MPEG_TS_)

The rationale for chosing that is:
* the namespace is shorter/direct (it's mpegts, not mpeg_ts nor mpeg-ts)
* the namespace is one word under Gst
* it's shorter (yah)