Hierarchical display of attributes in the job panel

[michaelpj]

Once one has a fair number of attributes being built (particularly if you are building across multiple platforms and nixpkgs versions), the jobs pane becomes a bit unwieldy.

Fortunately, we have some structure already there that we can make use of: the structure of the attributes themselves.

A nested, collapsible tree of derivations would be pretty nice. Probably there should be a UI toggle for flat mode, which is likely to still be nicer for small projects.