Child items show up in the source view (#7) and can be deleted.

Bones still do not show up, the UI for child items should not include all the scaling/positioning (but that needs to be reworked anyway), and the skeleton drawer remains easily confused.
Actually implement observing.
Bones from subitems show up correctly.

They are not hung in the subitem part, but rather in the main tree, but with the name of their parent in brackets. This needed code to dynamically update the bones that are present, and less caching of things that can suddenly change.
Fixes #7 by making skeleton drawer aware of the possibility of subitems.

And that's it for clothes support! In the future, it may also make sense to add a "separate" feature, to remove a child item from its parent but keeping all the existing parts, but that will be left to another time. Also, the UI needs improvements, but Item UI needs improvements anyway.
Fixed problem where skeleton drawer would smash the memory on addition of new objects

It simply did not now that bone counts could change without the
selection changing. That is now fixed.