playbin: don't show subtitles by default, or only if explicitly signalled to do

Submitted by Tim Müller @tpm

Link to original bug (#695968)

Description

See bug 690911.

If there are subtitles, they should probably not be enabled by default unless there is some signalling from the demuxer/file that they should be enabled.

Step 1: playbin should default to subtitles=off

As it says. Should be trivial to implement.

Step 2: let demuxers signal which streams are the default

We then need to figure out some signalling so that demuxers can tell playbin if one of the streams is the 'default' stream, and if so which. Could be done via by adding a flags argument to the stream-start event. See attachement 232879 of bug 600648 for a tentative patch.