Remove PSG from the repository

The PSG backend is broken and unlikely to be updated any time
soon.
Handle case when kerning is disabled
Fix bugs introduced recently and triggered by the rST example

- Line.new_span() may not be called from except handling section
  (as it raises exceptions itself)
- handle empty line case when removing a trailing space
Remove obsolete files

quickstart.xml was an XML representation of the rST node tree
produced by rst2xml.py
No need for storing a reference to the Document in every
DocumentElement

We can always reach it through the container (when rendering).
Prepare for named styles

- Only the styles of Flowables not constructed in __init__ are named
- Pass document to get_style()
Store cross-reference data in Document instead of in Referenceable
Create the Heading text only when on the initializing rendering state

- Only back-references (REFERENCE) will work
- Needs cleanup
Introduce a Paragraph superclass, ParagraphBase

- ParagraphBase does not store text
- Paragraph also inherits from MixedStyled to store text
- Heading, Header and Footer don't store text but generate it when
  its spans() method is called
Prepend rendering with extra step to set up cross-reference data
Don't require a Document reference on Heading initialization

- Also do not store generated unique IDs in Heading
- Breaks TableOfContents
Make GroupedFlowables more general (in preparation of TOC changes)
Generate table of contents just before rendering
Don't require a Document reference on Figure initialization

- Generalize InseparableFlowables
- Referenceable no longer requires a Document reference on init
- Do not store a Document reference in every Flowable anymore
Only show Heading number and separator if a numbering_style is set
Cleanup
Store only Referenceables without ID in Document.ids_by_element
Allow specifying List style by name
Allow specifying Tabular style by name (quick-n-dirty)
Allow specifying Caption style by name