Implement Pulse volume control as an API

[spiralofhope]

From #14 (comment)

Pulse volume control should be implemented through a proper API instead of parsing the output of shell commands as it is implemented now. You could probably fix the display of the Pulse volume easily, but if the output of pactl changes it would break again.

[xi]

PulseAudio recommends using libpulse rather than the dbus API. The way to go is probably the python library pulsectl. It uses libpulse via ctypes and seems to be well maintained. Since it uses libpulse, the interface is also very similar to that of the pactl cli.