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)
Support reStructuredText docinfo
Suppress reStructuredText generated

- section numbering is handled by RinohType
- fix Heading to accept a MixedStyledText title
Bugfix: correctly set page reference for sections
Previous page count and page references need not be attributes
Support reStructuredText contents

- do not use the table of contents generated by docutils, but use
RinohType's TableOfContents
- extend TableOfContents to allow generating a local table of contents

TODO: Styling for local TOCs needs to be specified explicitly for each
level. Make indent_left relative to the parent TableOfContentsEntry?
Support reStructuredtext definition classifier

Some cleanup:
- adjust Definition for StyleSheet styling
- allow passing a style to CustomElement.process_content()
Allow setting the PDF metadata

- set the Creator field on instantiation of the PDF Document
- the document title and other fields can be passed on Document
instantiation or at a later point in time by writing to the
corresponding attributes
Change the cache extension to 'rtc' (RinohType Cache)