Distinguish between Flowable/StyledText-yielding rST elements

- the build_flowable and build_styled_text methods make things explicit
- some elements are both BodyElement and InlineElement (e.g. Reference)
- for inline elements, build_styled_text is always called
- eliminate NestedElement; process_content needs to be available to both
  BodyElement and InlineElement
Handle text filtering in the frontend
Avoid deep nesting of MixedStyledText (only for rST)