dashdemux: forward availabilityStartTime to application

Submitted by Julien Isorce @cap

Link to original bug (#758952)

Description

The application needs a way to retrieve the "availabilityStartTime" field in mpd.

Indeed getStartDate method of HTML5 video tag shall return MPD@availabilityStartTime plus the PeriodStart time.

PeriodStart seems to be gst_adaptive_demux_get_period_start_time which should match start attribute in mpd, example: <Period .. start="PT0.00S">

One possible solution is to post a gst message. Should I add a new entry to GST_ADAPTIVE_DEMUX_STATISTICS_MESSAGE_NAME message: "start-date" ? Or a new message specific to dash where we can add as much as mpd's nodes/attributes that we need ?

(Currently with latest WebKit getStartDate always returns NaN since it is default implementation)