Skeleton mode: Show only selected bones and some context

[cochrane]

This is a new mode for the "skeleton" view, to solve the issue where there's just this massive confusion of yellow lines everywhere. The idea is to just show:

1. The selected bone (currently red)
2. Its immediate descendants (currently green)
3. Either only its parent, or everything from the root bone up to the selected bone (currently yellow)

I may have to implement both versions of point 3 and see which one I like best. This is particularly relevant for #81, where selection of bones doesn't happen in the document window anymore, to see what is going on.