screen for composing jams together

[cpmpercussion]

we need a separate screen for viewing jams by instrument/performer/time/other metadata and selecting ones to add together in a post-hoc composition.

[shredlocker]

Check out the composing branch!

[cpmpercussion]

This is sort of done, and now in the app, but it's currently been hidden as needs more testing and work. The biggest issue is how to store the many parents of a composed jam - the model, as it is, doesn't support a complex ancestor system. This could be changed, but requires more effort. Kicking to RC3