Non-blocking volume ramping

[roderickvd]

Feature Description

pleezer should quickly ramp the volume down, then up again, when seeking, skipping tracks, and stopping or starting playback.

Implementation Ideas

Rodio offers a periodic_access callback, not on the Sink but on each Source, although in our case only one source is active at any time. We could probably set the desired volume in an atomic variable or Arc and share it amongst the sources.

Alternatives Considered

The current Player::ramp_volume implementation in pleezer prevents the worst of popping and clicking sounds, but is very basic and blocking. It should be non-blocking.

[roderickvd]

v0.17.0 greatly improved volume ramping for volume control operations, but is still blocking. That it's blocking for the player is fine and in fact necessary: the player needs to know if a ramp is "complete" before it performs its next operation like changing songs. But maybe we can find a way to not block the thread.