Create message documentation

Created by: fred-wang

This issue is one important step towards issue #493 (closed). I'm about to push a qqq branch with the following changes:

• I created a fake qqq language with much more detailed message descriptions. I think it should be enough for translators, but perhaps some technical terms may need further explanations.
• I also wrote a new test/localization.html page that will allow localizers to quickly test the localized strings in context. I think almost all strings should be accessible from that test page by anyone, except perhaps some that depends on configuration like MathPlayer or Mac menu items. I was not sure how to get some of the error messages and so cheated a bit by inserting the text in an merror.

While writing that, I found a couple of bugs in the localization (both localization data and MathJax code). I haven't made a list, but it's certainly worth reporting them as other issue entries. They should be pretty easy to fix, the most involved one seems to be that HTML snippets in 'FontWarnings' don't have markdown URLs parsed (but are we really willing to keep the FontWarnings extension in the future?).