WIP: Reactive Visual tests and integration of Color with visuals

On a second thought, I will integrate Visuals with Color and then add the tests so that the changes can be included in few commits.
I don't like the idea of `Color.rgba` returning anything other than a `np.ndarray` with shape `(4,)`. Also, `(0, 0, 0, 0)` better not draw a visible black border because the alpha is zero :) It seems easiest just to implement a check `if self._border_color[3] != 0.` to know if the border should be drawn or not. (No reason to draw if alpha is zero.)