mpegts: ABI/API break: Use GPtrArray instead of GArray

mpegts: ABI/API break: Use GPtrArray instead of GArray

While it was a great idea, various g-i based bindings don't support
GArray with entries greater than sizeof(gpointer) :(

So let's just make everybody happy by just using GPtrArray.

And since we're breaking the API, also rename the various descriptor fields
to no longer have the descriptor_ prefix.

It does cost a bit more in terms of memory/cpu usage, but makes it usable
from bindings.