Add some tests exercising examples in README_cursorpositions.txt

Some failing examples have been commented out and left in place to raise
awareness of these position iteration issues.
Add some tests around paragraph split behaviour and direct styling

* failing test: paragraph split does not preserve span-level styling
Add failing text insertion scenario
Merge pull request #8 from peitschie/textposition-tests

Add some tests exercising examples in README_cursorpositions.txt