Display cover of first selected track

Display covers also if multiple tracks are selected. In that case we
default to the first selected track.
We must check if the list is not empty before using it anyway.
I pushed a new revision fixing it here:
https://github.com/cardinot/mixxx/commit/f47eb04d282904e85d0f2e92ea7d5f665ede2150
and it is also using the last index of the list.
if you find some issue in using this approach, we can back to use the first index.