Implement (partly) proper way to inherit from Entity

That is, each Subclass does:

    __init__(what=None, I=3, need=3, **kwargs):
        SuperClass.__init__(self, **kwargs)

I did not touch ModularVisual and their subclasses, as to not interfere with Erics MPL PR.

Also fixes to Text class and SceneCanvas can be other bgcolor than
black :)

Note that the text_visual example is broken, but I think this was already so.

closes #373
Maybe "The margin to keep outside the widget's border" for clarity wrt. `padding` ?