Add document model

[mjakeman]

• Reimplements the document model from bluetype relicensed to use the MIT licence.
• Simple RichTextWidget for displaying rich text documents
• Basic layout and font rendering
• Adds a simple demo application with rudimentary HTML parsing (only supports paragraph, line break at present) which is sufficient to display a comment from extensions.gnome.org.

It does not currently support:

• Formatting of any kind
• Any rich text element other than paragraph
• Caching and occlusion culling
• The ability to edit

TODO:

• Fix warning about text_paragraph_append_run not being defined
• Move parser into core library
• Move rich text widget into core library
• Fix include paths
• Pre-calculated layout
• Add unit tests Deferred (Unit Tests #4)