dashdemux: reworked the API to retrieve fragment timestamp, fragment duration and...

dashdemux: reworked the API to retrieve fragment timestamp, fragment duration and media presentation duration

This was necessary to support variable-duration Fragments.

in the new API:
- gst_mpd_client_get_current_position returns the timestamp of the NEXT fragment to download;
- gst_mpd_client_get_next_fragment_duration returns the duration of the next fragment to download;
- gst_mpd_client_get_media_presentation_duration returns the mediaPresentationDuration from the MPD file;

also there is a new internal parser function:
- gst_mpd_client_get_segment_duration extracts the constant segment duration from the MPD file
(only used when there is no SegmentTimeline syntax element in the current representation)

In gst_mpd_client_get_next_fragment, we set the timestamp/duration of the fragment just downloaded
copying the values from the corresponding GstMediaSegment.

TODO: rework SEEKING to support seeking across different Periods.