Chord/key analysis (Krumhansl-Schmuckler) — present in old slopsmith-plugin-editor, missing here

[carochacs]

Migrated from: got-feedBack/feedBack-plugin-editor#362

The old slopsmith editor had built-in chord/key analysis using Krumhansl-Schmuckler algorithm. This feature was present but has been dropped in the current feedBack editor.

Restoring this would enable users to analyze harmonic content and detect key/mode from transcribed arrangements.

See original issue for technical details and UI mockups.