Feature request: Support isomorphic layouts

Learning a midi keyboard with an isomorphic layout, like the Linnstrument, Lumatone, or a number of grid controllers supporting various isomorphic layouts, is difficult and there are not really any free and open source applications out there which will help you learn to play one.

Neothesia looks like one of the better free "midi-hero" style teaching apps, and it would be great if it had support for these odd-ball layouts for those of us who want to learn to play these instruments.

Since the "falling notes" layout only really works with a keyboard that has a single row of keys, maybe a way to support these types of layouts could be a key filling up in color and the color fading in, with a visual indicator of when it should be played.

Also most of these types of controllers support various layouts on the same controller, so being able to create a custom layout within Neothesia to correspond to your physical layout would be necessary.

The issue names no files, tests, or entry points. Start by locating the existing falling-notes and keyboard-layout paths, then define how custom isomorphic layouts and their visual play indicators should work before implementation.