Add support for selection priority in AdaptationSet.

[Lunkers]

Add support for the selectionPriority –added in ISO/IEC 23009- 1:2019– attribute to AdaptationSet. This attribute allows the MPD author to specify which AdaptationSet the player should choose when starting playback, for example:

two video codecs providing the same content, but one of the two provides higher compression efficiency and is therefore preferred by the MPD author.[1]

Or when audio is available in multiple languages, but the MPD author wants the player to select a specific language when playback starts.

This attribute is needed when providing multiple adaptation sets of the same contentType, such as alternate audio or video tracks. From the implementation guidelines:

If [@selectionPriority is] not present or non-differentiating values are provided, then the content author should expect a random selection of Adaptation Sets in case it is able to handle multiple Adaptation Sets within one media type and alternative content.[1]

[1]: Guidelines for Implementation: DASH-IF Interoperability Points November 15, 2018 DASH Industry Forum, version 4.3