adaptivedemux2: Playback hangs with VTT fragments

If the stream contains VTT fragments (i.e. HLS), playback will just hang.

The reason is that subparse is no longer plugged into parsebin (since !6153 (merged) ), and therefore the outputted buffers have no timestamps which causes havoc with the internal track level estimation.

The solutions are:

• Create an actual parser (could be based on subparse code itself).
• Do the minimal parsing within hlsdemux2 parsing code (i.e. return the properly timestamped buffers)

This might also be causing regression with other uses of parsebin (including in urisourcebin/uridecodebin3) and stand-alone VTT/subtitle files

Note: The issue does not happen if subtitles come from a container format.