Added notes

This version includes the major missing feature blocking it being possible even in the abstract to input Schenkerian analyses into the app format - implied tones. In short, one can now add new notes in the analysis layers (i.e. not the topmost), using a specific mode. Accidentals are not handled at the moment, and the drawing is somewhat basic, but the functionality is there.

Additionally, the old codepath has been excised, bringing the codebase into better shape for future features.

There are also, as usual, a number of additional improvements, bug fixes, refactorings, and so on, including

• A better notification of what elements are selected (thanks @yrammos)
• A new basic test for reductions and undoing reductions.
• The delete code has been moved to its own file, and now works also on metarelations
• Libraries are now in lib/ instead of cluttering up js/
• Additional documentation, and general cleanup of cosmetics and indentation
• MIDI playback of reductions are now per view