Change Duet logic so Performous doesn't crash anymore.

This changes the internal logic a bit: Since the Engine always wants the
same number of Analyzers as Vocals, we always use as much Analyzers as
vocals. In the case the user either selects "Normal Mode" from the Duet
menu or the song does not contain a duet in the first place, we just
assign the first selected vocal track (or the only one) to all
microphones.

TODO: The duet menu should somehow visualize when "Normal Mode" is
selected, that only the first vocal selection is relevant.
Show microphone icons in duet menu.

This still needs a little bit of layout adjusting...
Move microphone colors into a central place.

These colors were defined in 3 places. Does this need a more efficient
implementation?
Make microphone icon a bit smaller.
Fall back to naming the merged track "Together"...

Sometimes there are no usable artist names in Singstar's XML files. If
this is the case for a duet track, we call the merged track "Together".

Missing: <TRACK Name="Player1"> </TRACK>
All OK:  <TRACK Name="Player1" Artist="Elton John"> </TRACK>
Fix noteMin and noteMax of combined duet track.
Merge pull request #74 from performous/duet-fixes

Duet fixes
desktop file
Apparantly desktop file needs chmod +x.
Merge remote-tracking branch 'origin/master' into sdl2