Custom PulseAudio max volume

Allows the user using PulseAudio to specify a custom max volume value through the gsettings schema interface instead of the hard-coded MAX_VOLUME specified in the source code.

Commits a9bfbd6 and b4044c3 only occur under pulse, while c4e2a39 affects both pulse and gstreamer.
Tested with both pulse and gstreamer backends.