Table cells are insets, but not from the point of view of DocIterators

Table cells are insets, but not from the point of view of DocIterators
and Cursors. So just calling InsetText::addToToc for the cells causes
problems, because InsetText::addToToc then adds the cell inset itself
as part of the DocIterator. This then leads to assertions, such as bug

The solution is to refactor InsetText::addToToc so that we can call the
iterating part without adding the inset.