Mic try2

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 (a sepatate commit in this PR) 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).

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

Try 2: Some code consolidation by Martin and Gonzalo; rename
speaker.py to audio.py