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.