Mic levels (fixed SL #800)

Add control for microphone level setting

This patch adds a microphone level and mute control to the speaker device on the Sugar Frame.
It makes use of an extension to the sound model by Emil Dudev and a new icon, media-audio-input.svg
in icons/scaleable/devices. (The extension to the sound model also requires a change in
sugar-toolkit-gtk3 sugar3/acme-volume-alsa.c and sugar3/acme-volume-alsa.h).

Fixes SL #800

See [1] and [2] for the Design Team discussion.
    
    [1] http://wiki.sugarlabs.org/go/File:MicLevels.png
    [2] http://lists.sugarlabs.org/archive/sugar-devel/2014-March/047640.html