Add 1.6 VideoModel changes

[LloydW93]

This PR reintroduces changes in the VideoModel that were not present in 1.5.1.

[LloydW93]

There are a few parts to these changes:

• stalledStreams had two different ways of tracking each stalled stream, in both list and object form. We've removed the use of the object form (stalledStreams.audio, stalledStreams.video) in favour of the list form, and also added checks to prevent the same stream being pushed in as stalled twice.
• setSource as per discussions at the time, will cause the user agent to error due to an unsupported source if something falsey is passed in. This is currently done as part of reset (see [2.0] reset() has weird behaviour #974), which does make the line useless, which could be a little confusing.
• previousPlaybackRate enables clients to set a custom playbackRate on the media element (e.g. 0.5 or 2) and not have this reset on rebuffering.
• The playing/waiting events are to enable clients to see more natural events from the media elements. In our use case, our UI listens to the playing/waiting events on the media element to show a rebuffering indicator. This enables us to use the same events from the same object when dash.js pauses playback rebuffering.