[Transitions] Full support

[beefchimi]

At the moment, transitions only apply to volume / mute changes.

To achieve full support, transitions would also affect:

• play (quick fade in).
• pause (quick fade out).
• stop (just like with pause).
• speed (ramp the speed change)
• reverse (ramp speed to 0 before changing directions, then ramp back up to the saved playbackRate).
• seek (ramp speed to desired play position).

All of these features come with some complexity... some more than others. So it is debatable if we really want to go down this path.