Timeline audio playback

r269 introduces audio tracks to be played in timelines. There are a few caveats which are not obvious about them. Here they are:

1. The Audio plugin needs to be included in the project. Timelines playing audio use it under the hood, but they do not enforce it's existence in the project.
2. A timeline can not play audio until there has been any meaningful user input. A single click is enough, but if there is no user input, no audio will be played.
3. For better results, make sure the audio that is going to be played by a timeline is preloaded before the timeline starts playback.
4. If a timeline is played backwards by using a negative playback rate, audio will not be played. A timeline will only trigger audio playback when playing with a positive playback rate.
5. Audio tracks are created with the default audio source property track, but it is also possible to add an optional volume property track by right clicking on the track and selecting the Add properties context menu option.

I think those are all the less obvious details.