icon for audio input

Of general utility, but specifically needed for the microphone level
adjustment patch. Based on the microphone icon used in the Measure
activity.

(Part of a series of patches to fix 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