Returns a list of W(TextTag) that are toggled on or off at this point.
(If toggled_on is true, the list contains tags that are toggled on).
If a tag is toggled on at this iterator, then some non-empty range
of characters following this iter has that tag applied to it. If a
tag is toggled off, then some non-empty range following this iter does
not have the tag applied to it.