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-input-video.conf

@@ -0,0 +1,31 @@
+# For devices, where we have a Video element
+
+[General]
+priority = 70
+
+[Element Capture]
+switch = mute
+volume = merge
+override-map.1 = all
+override-map.2 = all-left,all-right
+
+[Element Mic]
+switch = off
+volume = off
+
+[Element Line]
+switch = off
+volume = off
+
+[Element Aux]
+switch = off
+volume = off
+
+[Element Video]
+required = any
+switch = mute
+volume = merge
+override-map.1 = all
+override-map.2 = all-left,all-right
+
+.include analog-input.conf.common