re #428: Push an undo snapshot when updating formula.

Explicitly push a snapshot onto the CKEditor undo stack when updating a formula
from the WIRIS editor; we don't seem to get one of these otherwise when editing
an existing formula.

This serves the dual purposes of simply having another undo/redo point available,
and having a suitable event to listen for where we need to know when content has
been changed other than by typing.