Re-add the shadow cursor's selection overlays on document undo

This is an interm fix until a "grown-up" undo stack arrives.

The current undo implementation for single user replaces the document content, which causes OdfCanvas to entirely clear and replace all children in the parent element.

Most overlays are unaffected as they are re-added when the cursor is redraw. The shadow cursor is not an added cursor however, and so after an undo it stops functioning.