Add new icon for Preferences > Key Detection

The current icon (a single bar note) is replaced.
Why a new icon?
* It can not be mistaken for with the existing iTunes icon.
* it has a distinctive, but still familiar shape in the look of the existing icons.
* The imagery of the icon is coherent with its message.

It consist of a clef and bold double barline. A clef is a musical symbol used to indicate the pitch of written notes, while a bold double bar line marks the end of a track. IMO it sums up exactly what key detection does - estimating  the musical key of a track.

Note: I cant test the changes because the required rubberband library wont compile on OSX (not it macports nor homebrew).