#31 Undo/redo on item being edited doesn't update editor

[zw]

Edit an item, and make some change, and click "Done".
Open the editor again and make another change, but leave it uncommitted.
In the main window, choose Edit->Undo. Notice that the editor doesn't change.

Bit of a tricky one, as with most bugs arising from the ability to edit multiple items at once. The changes made by the undo may conflict with non-writeBack()ed changes in the proxy.

Each item could track its active proxy (this assumes there's only ever one), which could then be checked in the undo; if there is one and it's modified, prompt the user. Explain that undo/redo will overwrite local edits. If there's a proxy but it's unmodified, silently update the VP in place. Possibly worth noting that once we fix 1774767 the editor will have its own undo menu (or on OS X, will install its own items when it has focus), so focus must currently be on another frame for the higher-level "Undo" to be reachable.