Output correct HTML entity for \r{a}

There is an issue with characters å and Å (with ring above). The corresponding HTML entities are &aring; / &Aring;

To get this character with latex we'd do: \r{a} or \aa

With bibtextbrowser \aa works fine. However, \r{a} doesn't do anything at all. This pull fixes this, we now output the correct HTML for \r{a} and \r{A}.

Furthermore, up till now bibtexbrowser will output &aring; if it encounters \.{a}, but this is wrong since this latex is meant to be used to produce an overdot (dot above char) and not "ring above char". Since in practice this is a "spelling error", this behaviour is removed and from now on \.{a} will not get replaced.