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
Actually, I would get rid of it entirely since it's taken care of by the `gloo.clear()` below. Or you can set it here, use `color=True` below, *and* add something that sets it when the user sets the `bgcolor` parameter (harder b/c you can only do it once it's initialized) -- +1 for just using `gloo.clear(color=self._bgcolor, ...)` for simplicity