volume: Add LFE balance API

volume: Add LFE balance API

The gnome/unity-control-center UIs have a master volume slider, and
three sub-sliders: balance, fade, and subwoofer. Balance and fade
use PA's set_balance and set_fade APIs accordingly, but the subwoofer
slider sometimes does unintuitive things.

In order to make that slider behave better, let's add a LFE balance
API that these volume control UIs can use instead. With this API,
the UI can balance between "no subwoofer" and "only subwoofer" with
"equal balance" in the middle, which would make it more consistent
with the behaviour of the other sliders.

BugLink: https://bugzilla.gnome.org/show_bug.cgi?id=753847
Signed-off-by: David Henningsson <david.henningsson@canonical.com>