The Markdown UI's line generator for rendered source didn't handle lists

very well, and had some other less-than-ideal rendering issues. We now

have a more specialized line emitter for rendered Markdown, which has

been moved to markdown_utils.py for use by other code.
On top of that, source was rendered using some non-Markdown lexer.

Pygments, it turns out, doesn't have a Markdown lexer, oddly.

ReStructured Text is really close, but not a complete match, and could

lead to some confusion. So, we're using the plain-text renderer for now.

Tested with a Markdown file using lists, code blocks, headers, and other

data.
Also used these with additional testing for a new UI making use of Markdown

line iteration.