The original intention of this code was probably that if
adding filter1 succeeded but adding filter2 failed, then
filter1 should be removed so that either both or none of the
filters get added.
We didn't do anything anyway in case of failures. When we
give NULL as the error, dbus_bus_remove_match() can act
asynchronously, so it becomes faster. Also, the bus daemon
can avoid sending any replies, which reduces the amount of
traffic.
Completely rework mixer logic. This now allows controlling a full set of
elements from a single sink's volume slider/mute button.
This also introduces sink and source "ports" that can be used to choose
different input or output ports with the UI. (i.e. "mic"/"line-in" or
"speaker"/"headphones".
The mixer paths and device maps are now configered in external
configuration files and can be tweaked as necessary.
