Edward Hervey authored Nov 10, 2017 and Edward Hervey committed Feb 10, 2018

Similar in vein to the playbin2 architecture except that uridecodebin3
are prerolled much earlier and all streams of the same type are
fed through a 'concat' element.

This keeps the philosphy of having all elements connected as soon
as possible.

The 'about-to-finish' signal is emitted whenever one of the uridecodebin
is about to finish, allowing the users to set the next uri/suburi.

The notion of a group being active has changed. It now means that the
uridecodebin3 has been activated, but doesn't mean it is the one
currently being outputted by the sinks (i.e. curr_group and next_group).
This is done via detecting GST_MESSAGE_STREAM_START emission by playsink
and figuring out which group is really playing.

When the current group changes, a new thread is started to deactivate
the previous one and optionnaly fire 'about-to-finish'.