reStructuredText can also have content outside of sections

- handle Title differently depending on context
- Note is similar to Tip
- suppress docinfo and topic (table of contents) for now
Implement sections as StaticGroupedFlowables

- Section now is the Referenceable, not Heading
- every Flowable now has a level
- build the table of contents based on document.elements
- provide a default source for each DocumentElement

Advantages:
- building the Flowables from a (XML or rST) document tree is easier
- more information is contained in the RinohType doctree
- and so more styling can be applied (e.g. indentation of sections)

Breaks (RFIC example):
- reflow on float placement (Figures section)
- ToC and References are included in the table of contents
Fix reflow on Float placement