Basic support for reStructuredText target elements

Target elements can both occur as body and inline elements.

Body target:
- when the element has no content
- output a DummyFlowable for now

Inline target:
- output MixedStyledText
Pass the last descender to the next footnote
Support all reStructuredText footnote types

Need to fix: footnotes in references from footnotes render on top of
each other
Footnote rendering fixes

- don't render footnotes twice
- fix rendering of footnotes referenced by other footnotes

A reference to the same footnote from within a footnote leads to
infinite recursion.
Some minor style improvements
Refactor Heading numbering
Use the descriptive 'path' property instead of __str__

__str__ is used in MixedStyledText to return the text content
(and could cause infinite recursion in some cases)
Typeset the reStructuredText option argument delimiter
Note now is a Referenceable / NoteMarker is a Reference

Numbering is not yet implemented. The Note's label needs to be
generated at a later point in time (as in Heading).

reStructuredText frontend:
- no more need to search the rST doctree for the footnote
  corresponding to a footnote_reference
- no more infinite recursion when a footnote references itself

XML frontend:
- the note can be passed along with the marker (no need for an ID)
Add a new symbol number format
Refactoring of number styling

- introduce NumberStyle class, subclassed by Heading/ListStyle
- rename 'numbering_*' to 'number_*' style attributes
Refactor Style._supported_attributes()

Collect the supported attribute names in a set.
Fix retrieval of default value for a style attribute

The default value was retrieved from the style class associated with the
top-level parent instead of from the Styled's style class.
A Reference is a StyledText, thus should have a PARENT_STYLE by default
Adapt sub/superscript code to the "recent" StyleSheet changes
Number NoteMarker and allow styling them