- build a list of the available Periods with their start and duration time
- add the list of GstStreamPeriod in the GstMpdClient data struct
- remove cur_period from GstMpdClient and introduce an API to get the current GstStreamPeriod
- several API clean-ups
build the list of segments to be played using the SegmentTimeline syntax, if present
bugfixes:
- for dynamic MPD files, when mediaPresentationDuration is not present use minimumUpdatePeriod instead
- do not add a spurious '$' when building an URL from a template like "$Bandwidth$/init.mp4v"
- introduce gst_mpd_client_add_media_segment() to avoid code duplication
SegmentList nodes are allowed into Period, AdaptationSet or Representation nodes
and there is at most 1 element, so no need to keep a list;
Period nodes cannot have any Represention elements, as AdaptationSet nodes are mandatory;
this breaks compatibility with some legacy DASH test sequences.
