Allow better control of the audio volume
Similarly with how the x/y/w/h props of a video clip are displayed in the Clip Properties panel, we should have the volume of an audio clip editable as a numeric value.
Note there are two cases, depending on whether the audio volume is "uncontrolled" [by keyframes]:
- uncontrolled: there is a single value applied to the entire audio clip;
- controlled: enabling/disabling this should be done similarly to how we do it for video files..