The delete_range signal is emitted to delete a range from a TextBuffer.
Note that if your handler runs before the default handler it must not invalidate the
end iters (or has to
revalidate them). The default signal handler revalidates the
end iters to both point to the location where
text was deleted. Handlers which run after the default handler (see
connect_after) do not have access to the deleted text.
See also: @delete.
the start of the range to be deleted
the end of the range to be deleted