Moved the region's add method to the general container abstraction since such
functionality is useful for other nodes such as table cells provided by the
table parser.
1 <<TableOfContents(1, 2)>>
2 3 = Heading =
4 5 Some text. <<TableOfContents(2)>> And more.
6 7 == Subheading ==
8 9 More<<BR>>text.
10 11 Some <<Verbatim(''verbatim'' text)>>.