Is your feature request related to a problem? Please describe.

The alsa module is lacking middle and right button events.

Why does polybar need this feature?

The other audio module, pulseaudio, has them all, so it's really a simple matter of consistency.

Describe the solution you'd like

Adding the missing events to the alsa module.

Describe alternatives you've considered

N/T

Additional context

Since this is pretty much just copypaste from the pulseaudio module to the alsa one, I can open a PR myself.