[JEWEL-495] Fix nested inlines in Markdown

[rock3r]

Properly supporting nested inline styling in Markdown requires a lot of changes. Luckily, they're mostly internal, but they do require us to deprecate and hide the old APIs because the behaviour has changed.

(note how the `UI` part is italic, inheriting the emphasis, and the next paragraph also renders as expected now)

How does this fix work? Essentially, we need a smarter way to merge the styling as we visit nodes:

1. We carry the "current" text style with us, that we can merge additional styles into
2. We change the inline styles to only carry the minimum amount of style changes, so that we can them stack-merge them as we visit, without overriding important properties
3. We have a custom smartMerge algorithm, used in the default inline renderer, that works the way we'd expect it to for Markdown

In addition to that:

• We've removed the renderHtml property from the inline Markdown, since it's weird that would override the renderer; now you can actually decide it in the inline renderer
• Default to render the inline HTML, as disabled-looking monospace, so that no information is lost during render
• In the default inline renderer we've extracted rendering functions so that other implementations can replace one or more of them easily