Use <quote> tags in XML output

This is to mark strings for formatting rather than picking a particular
formatting upfront.

Author: tautschnig

src/util/ui_message.h

@@ -131,8 +131,9 @@ class ui_message_handlert : public message_handlert
       case 96: // bright_cyan
         return std::string();
       case '<': // quote_begin
+        return "<quote>";
       case '>': // quote_end
-        return "'";
+        return "</quote>";
       }
       break;
     case uit::JSON_UI: