Draft: Make line breaking available outside PangoLayout

This branch addresses #391 (closed)

It does the following things:

• Split PangoLayout into multiple objects:
  • PangoSimpleLayout, the frontend object, holding layout properties
  • PangoLineBreaker, for doing the core line breaking
  • PangoLine, an immutable, opaque version of PangoLayoutLine
  • PangoLines, for holding positioned lines
  • PangoLineIter, for iterating over PangoLines

I think what I would like to do ultimatively is to drop the current PangoLayout and rename PangoSimpleLayout to PangoLayout. Feature-wise, PangoSimpleLayout covers all that PangoLayout does. There will be some slight api changes and cleanups, but nothing too dramatic:

• PangoAlignmentMode unifies alignment and justification
• PangoWrapMode unifies wrapping and single-paragraph-mode
• spacing was dropped (for now, may come back)
• all of the api related to formatted output, including editing and cursor positioning, was moved to PangoLines
• all of the api related to iterating was moved to PangoLineIter
• PangoLayoutLine was replaced by PangoLine

The main reason for the wrap mode change was to keep the argument list of pango_line_breaker_next_line short, but maybe that isn't right.

The line-spacing factor was reimplemented as just a wrapper around the line-height attribute.