Re-add the shadow cursor's selection 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.