Custom chord variants

[PetrIvan]

Instead of just some fixed chord variants, add a possibility for users to add their own. It would be probably done by clicking on the piano keys in the chord variants, notes that are not a part of that chord wouldn't be possible to select. Users could then name the chords however they like or just default to the original one (e.g. a custom variant of Gmaj7 would be named just Gmaj7 by default).