Gets a field by its "showname"
        (the name that appears in Wireshark's detailed display i.e. in 'User-Agent: Mozilla...', 'User-Agent' is the
         showname)

         Returns None if not found.

def get_field_by_showname(self, showname):
        """
        Gets a field by its "showname"
        (the name that appears in Wireshark's detailed display i.e. in 'User-Agent: Mozilla...', 'User-Agent' is the
         showname)

         Returns None if not found.
        """
        for field in self._get_all_fields_with_alternates():
            if field.showname_key == showname:
                # Return it if "XXX: whatever == XXX"
                return field