Add common playback shortcuts like in most other video players (VLC, YouTube, etc.)

[porky11]

Feature description

I'd like these controls, which are pretty common to video players:

• Space: Pause/Resume audio book
• Left/Right arrow: Back and forth a few seconds (maybe continuously by keeping it pressed, maybe higher or lower steps by pressing shift/ctrl)
• Up/Down: Change volume

Current state

Currently it already works a little.
Space works as expected (most of the time only?).
Arrow keys do different things depending on the selection.
Usually it's possible to select different UI elements using the arrow keys, which doesn't seem like a good feature at all, since as soon as one of the sliders (time, volume) is selected, it's not possible to select different UI elements using the arrow keys (likely a backend issue).

When one of the sliders is selected, two buttons always do the same action (up and left to move up OR left, right and down to move right OR down).
Space still works as expected.

So when one of the sliders is selected, it almost works as expected.
Like when the time bar is selected, left and right allows me to move back and forth a few seconds, while space is still able to pause/resume the playback.
But then I think, I want it to be a little louder, and press up, as I'm used from most video players, and insdead I move back in time, which is pretty annoying.