update key and mode selection interface

[jacobroufa]

Currently, in the New Song form, the key and mode selection uses a MultiSelect. Ideally, there should be a way to have a single scale entry with two selects (key and mode), and an "Add another scale" button, to add unlimited scales.

Specifically what we're looking to get rid of here, is in the case where I want to select Bm and D, for instance. As it stands now if you select "b" and "d" in the "keys" MultiSelect, and "major" and "minor" in the "modes" MultiSelect, you end up with four total scales, where you only wanted one.

[jacobroufa]

This is mostly complete as of 039fbfa.

I would like to ensure that when the interface has been closed, the number of scales is set back to 1 for the next time the form opens...