Refactor property editors to work on composites, lists, etc.

[adamkewley]

The available property editors (contained in src/OpenSimBindings/Widgets/ObjectPropertiesEditor.cpp) currently, for the most part, only handle singular values. The OpenSim API also supports optional and list values. The property editor should be updated to support this.

Doing this will probably involve injecting a PopupHost into the ObjectPropertiesEditor, so that it can (e.g.) open a popup that the user can use to edit the list (add elements, rearrange it, etc.) - therefore it's a more advanced change.