alsa: rework mixer logic

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.

src/modules/alsa/mixer/paths/analog-output-headphones.conf

@@ -0,0 +1,53 @@
+# Path for mixers that have a Headphone slider
+
+[General]
+priority = 90
+
+[Element Hardware Master]
+switch = mute
+volume = merge
+override-map.1 = all
+override-map.2 = all-left,all-right
+
+[Element Master]
+switch = mute
+volume = merge
+override-map.1 = all
+override-map.2 = all-left,all-right
+
+[Element Master Mono]
+switch = off
+volume = off
+
+[Element Headphone]
+required = any
+switch = mute
+volume = merge
+override-map.1 = all
+override-map.2 = all-left,all-right
+
+[Element Front]
+switch = off
+volume = off
+
+[Element Rear]
+switch = off
+volume = off
+
+[Element Sourround]
+switch = off
+volume = off
+
+[Element Side]
+switch = off
+volume = off
+
+[Element Center]
+switch = off
+volume = off
+
+[Element LFE]
+switch = off
+volume = off
+
+.include analog-output.conf.common