dashdemux: Rewrite ISOBMFF & SIDX handling

dashdemux: Rewrite ISOBMFF & SIDX handling

The previous code was handling both as separate steps and then tried to
combine the results, but this resulted in all kinds of bugs which showed
themselves as failures during seeking and offset tracking getting wrong.
This also showed itself with gst-validate on the sample stream.

The rewritten code now parses everything in one go and tracks the
current offset only once, and as a side effect simplifies the code a
lot.

Also added is detection of SIDX that point to other SIDX instead of
actual media segments, e.g. with this stream:
  http://dash.akamaized.net/dash264/TestCases/1a/sony/SNE_DASH_SD_CASE1A_REVISED.mpd
Support for this will have to be added at some point but that should
also be easier with the rewritten code.

https://bugzilla.gnome.org/show_bug.cgi?id=781233