After editing an image multiple times (e.g. rotate) and then undoing,
the thumbnail displays a previous edit state between the undos.
This is caused by connecting to QUndoStack signal indexChanged
which emits before some undo operations are finished and the new
thumbnail is created too early.
This patch emits the modified/saved signals for a document only
after the undo image operation is finished. It also solves the problem
displaying a wrong thumbnail in another mode after undoing all edits.
Depends on D11714