Player is missing volume control

I like minimalism, but a player without a volume control is kind of useless.

Sure, I can control the whole PC's volume, but is that what I want to do..?

The issue names no files, tests, or entry points. First locate the player controls and playback entry points, then clarify the intended volume behavior with maintainers; done means the player provides a usable volume control without changing the whole system volume.

Written by the indexing model from the issue text.