Add Attachment and Complex Character Positioning

[mjakeman]

• Adds 'attachment' - this is a term borrowed from webkit to describe the process of attaching semantic model nodes to view-specific layout nodes.
• Implements home and end line traversal according to Formalise character positioning #17
• Implements mouse-based picking for determining index under mouse position

TODO

• Add up/down movement
• Add cursor selection
• Add unit tests for home/end Deferred until traversal is separated from view
• Issues:
  • Fix issue with splitting then entering at the start of the paragraph
  • Cannot select final index in paragraph with mouse Pango issue, deferred until resolution on use of paragraph break characters
• Clean up:
  • Rework cursor drawing to only reallocate when necessary
  • Use fuzzy cursor picking (i.e. choose nearest when no exact match is found)
  • Up/down within paragraphs