Tree- and ViewModel out of sync after MultiValue change Undo

When the Value content of a MultiValue is changed, the first Undo works, but the second Undo after this one will lead to a disparity between GtkTreeView and GtkTreeModel resulting in a broken view for the user until a different section is selected.

Start by reproducing the issue in the odML GUI editor: change the Value content of a MultiValue, perform Undo twice, and observe the GtkTreeView and GtkTreeModel. Trace the undo handling involved in that sequence and verify that the view remains synchronized after the second Undo.

Written by the indexing model from the issue text.