dvdnav: implement GstStream API to allow switching DVD angles in playbin3
Submitted by Bastien Nocera @hadess
Link to original bug (#615060)
Description
When watching a DVD, we should be told when the number of viewing angles changes through the "n-video" playbin2 property and allow switching between angles through it as well.
That would make it possible to make the "Next Angle" button unsensitive when there's only one angle available.