gst-libs: New Mpeg-TS support library

gst-libs: New Mpeg-TS support library

Exposes various MPEG-TS (ISO/IEC 13818-1) and DVB (EN 300 468) Section
Information as well as descriptors for usage by plugins and applications.

This replaces entirely the old GstStructure-based system for conveying
mpeg-ts information to applications and other plugins.

Parsing and validation is done on a "when-needed" basis. This ensures
the minimal overhead for elements and applications creating and using
sections and descriptors.

Since all information is made available, this also allows applications
to parse custom sections and descriptors.

Right now the library is targeted towards parsing, but the structures
could be used in the future to allow applications to create and inject
sections and descriptors (for usage by various mpeg-ts elements).

https://bugzilla.gnome.org/show_bug.cgi?id=702724