Add editing interface

[mjakeman]

Enables the user to manipulate the document model

• Adds new TextDocument object
• Encapsulates editing functionality into TextEditor
• Supports linear traversal over a tree structure
• Simple reliable cursor support
• Insertion, deletion, and replacement
• A TextMark structure which represents a position within a document

TODO:

• [X] More robust movement
• [x] Use per-paragraph indexing
• [x] Return or split functionality
• [x] Fix crashes involving selections/replacement
• [x] Handle start/end iters in both directions
• [x] Generic way of updating/invalidating mark locations
• [x] Visually indicate selections Deferred until layout/style rework
• [x] Grab focus on click
• [x] Unit tests (Part of #4)
  • [X] insert
  • [x] delete
  • [x] replace
  • [x] movement
  • [x] split
  • [x] mark

[mjakeman]

Design doc for caret positions and indexing: