Generic handler for creating custom widgets, internally used to
        enable custom widgets (custom widgets of glade).

        The custom widgets have a creation function specified in design time.
        Those creation functions are always called with str1,str2,int1,int2 as
        arguments, that are values specified in design time.

        Methods of classes inheriting from SimpleGladeApp are used as
        creation functions automatically.

        If a custom widget has create_foo as creation function, then the
        method named create_foo is called with str1,str2,int1,int2 as arguments.

def custom_handler(self, glade, function_name, widget_name, str1, str2, int1, int2):
        """
        Generic handler for creating custom widgets, internally used to
        enable custom widgets (custom widgets of glade).

        The custom widgets have a creation function specified in design time.
        Those creation functions are always called with str1,str2,int1,int2 as
        arguments, that are values specified in design time.

        Methods of classes inheriting from SimpleGladeApp are used as
        creation functions automatically.

        If a custom widget has create_foo as creation function, then the
        method named create_foo is called with str1,str2,int1,int2 as arguments.
        """
        try:
            handler = getattr(self, function_name)
            return handler(str1, str2, int1, int2)
        except AttributeError:
            return None