This also will add mouseovers with the original text.

We already ignore the immediate children of script, form,
etc. elements, as well as spans that we've inserted ourselves.  This
change makes us skip the entire subtree of such an element.

One important effect is that if another extension changes one of our
spans using a similar algorithm to ours (inserting a new subtree) we
previously could end up reprocessing that node in a future
document_end event.  Now, we prevent that.

This also changes the disabled icon, since it has a drawn-in badge.
If it makes substitutions but then disables the extension, then the
substitution count is still shown until the page is reloaded (because
there are still substitutions on the page).  The two badges conflict
visually.