dashdemux: Add support for DASH-IF Trick Modes

Submitted by Wojciech Przybyl

Add support for Trick Mode play as per DASH-IF Interoperability v3.3 point: 3.2.9.

Use Adaptation Sets with a Essential Property set to:
"URI: http://dashif.org/guidelines/trickmode"
as Adaptation Sets for Trick Mode playback only.