Output correct HTML entity for \r{a}

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

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 å 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.

bibtexbrowser.php

@@ -879,7 +879,7 @@ function latex2html($line) {
   $line = char2html($line,'^','o',"circ");
   $line = char2html($line,'^','u',"circ");
 
-  $line = char2html($line,'.','a',"ring");
+  $line = char2html($line,'r','a',"ring");
 
   $line = char2html($line,'c','c',"cedil");
   $line = char2html($line,'v','s',"caron");