#97 undo after resolve all instances destroys scene

[Randolf Schultz]

When the undo or redo operation is performed after
the "resolve all instances" operation, the scene may
be destroyed in a subtile way: if instance objects that
are resolved by "resolve all instances" are in the undo
buffer; after the undo operation, their type id will be
changed back to instance, but they are of course not
instances anymore.