qmdiEditor: better markdown preview

[diegoiast]

I am trying to make a nice Markdown document, but lets be honest - QTextBrowser is not ideal. Lets instead - convert the markdown to HTML, and then feed it to a proper HTML widget. I don't think using the web engine from Qt is the best idea.

Instead - lets use:

1. https://github.com/litehtml/litehtml - this small engine is capable to display pretty decent HTML5.
2. https://invent.kde.org/qt/playground/qlitehtml - seems like there is a Qt port (is this basically LGPL? I prefer MIT licences).
3. https://github.com/progsource/maddy - we can convert the Markdown to HTML with this code
4. Now we only need to find a cool CSS for the generated HTML:
   1. https://github.com/sindresorhus/github-markdown-css
   2. https://github.com/dbohdan/classless-css#markdown-css--air
   3. https://github.com/mixu/markdown-styles
   4. https://benjam.info/panam/ (?)
   5. https://github.com/mixu/markdown-styles
   6. https://github.com/markdowncss
   7. Probably more