Pending fixes: enumerate valid markdown syntax inside link texts and captions

[s-cerevisiae]

Currently only Text, InlineMath and Code are allowed, leading to minor but annoying problems like SoftBreaks in links are ignored.

There should be clearly-defined rules of which events are allowed inside link texts and captions respectively. We can allow more events and produce warnings when encountering erroneous ones.

Furthermore we need to have correct transformation (to html) methods in these contexts. process::content::State::raw_text might be a good reference (and is probably reusable).