Improve markdown documentation rendering by using a new markdown parser

Fixes #1478 (closed)

This MR replaces the old gs_markdown parser/converter with a new markdown converter based on cmark, which converts the markdown to pango markup as well as possible.

Pictures

Old:

New:

Remaining problems:

• Images: not displayed
• Thematic break (aka horizontal rule): displayed as empty line
• Quotes, html: displayed as normal text
• Code blocks: no syntax highlighting

IMO, this is already a good start despite the limitations.

I could use help with a few details, so this is still WIP.

I'm also wondering if converting to a full gtk widget tree wouldn't be a better approach, which would let us e.g. display images, thematic breaks, or let us use a sourceview widget or similar to get syntax highlighting for code blocks, but I'm not sure if there aren't any problems with that.