add script command for menu items with alternate fonts

Also: add '\a' to the list of escaped characters and remove storage of
  unneeded "filename".

mitemaltfont <altfont> <text> <action> <hoveraction> <desc>
  Displays a menu line with a text which may contain chars from an
  alternate font.

  altfont
    Name of the alternate font.

  text
    Content of menu line. Chars from the alternate font have to be marked
    with "\a". Alternate font chars have no width and are just rendered
    above the following text. Use spaces to counter that.

  action, hoveraction, desc
    Same function as with other menu lines.

DOCREF menuitemaltfont

source/src/protocol.cpp

@@ -246,6 +246,7 @@ void filterrichtext(char *dst, const char *src, int len)
             {
                 case '\0': --src; continue;
                 case 'f': c = '\f'; break;
+                case 'a': c = '\a'; break;
                 case 'n': c = '\n'; break;
                 case 'x':
                     b = 16;