translator: added support for the Apertium translation service

Also added feedback to source & target language menus to indicate which
languages are supported by the selected translation method. For Apertium, the
source language menu shows all possible source languages, and the target menu
shows the possible target languages for the chosen source language.

src/www/translator/translator.css

@@ -27,6 +27,8 @@ table.menubar td:hover, table.menubar dt:hover, dl.popupmenu > dt:hover {
 }
 table table dl { left: 6em; }
 table.menubar dt { white-space: nowrap; }
+dt.unsupported { color: #999; }
+
 div.document {
     /*margin-top: 7ex;*/
     clear: both;