Implement hierarchical diff presentation for object shadows

[j4yk]

For object shadows (i.e. slot-based snapshots of arbitrary objects), there is only a stub implementation of hierarchical diffs (i.e. the conversion from diffs to tree nodes and vice versa). The conversion should be completed to include all related object snapshots.

Maybe create the tree lazily? But keep conflict nodes (#needsChoice) in mind.

[j4yk]

This is obviously similar to #145, but I am not sure right now whether they are actually the same. This issue has the trees in commit and load dialogs in mind, which happen to be manipulable (excluded changes, conflict resolution), while #145 is about general browsing of differences, which needs not consider manipulation from the UI.