Visualize distance to a key when pitch shifting

This pull request still has a problem. I don't know how to get the group name in WKey, so I hard coded it for testing purposes (https://github.com/badescunicu/mixxx/compare/feat_1258624?expand=1#diff-6d2574508abe43353167c52253d2049bR10).

As a consequence, it updates the visual_key widget for both loaded tracks with the value received from the first one.