add DictValue

Ren'Py doesn't have the BarValue which adjustments a dictionary.
And I added DictValue.

        class DictValue(BarValue):
            """
             :doc: value

             A value that allows the user to adjust the value of a key
             in a dict.

             `dict`
                 The dict.
             `key`
                 The key.
             `range`
                 The range to adjust over.
             `max_is_zero`
                 If True, then when the value of a key is zero, the value of the
                 bar will be range, and all other values will be shifted down by 1.
                 This works both ways - when the bar is set to the maximum, the
                 value of a key is set to 0.

             `style`
                 The styles of the bar created.
             `offset`
                 An offset to add to the value.
             `step`
                 The amount to change the bar by. If None, defaults to 1/10th of
                 the bar.