Implement (partly) proper way to inherit from Entity

That is, each Subclass does `__init__(what=None, I=3, need=3, **kwargs)`
and then `SuperClass.__init__(self, **kwargs)`

Also fixes to Text class and SceneCanvas can be other bgcolor than
black :)
Address reviewer comments

I incorporated changes from #358, so that when that PR gets
rebased, it should result in minimal clashes.
flakify
Make ipynb_vnc not tested
address comments
Merge pull request #378 from almarklein/entity-inheritance

Implement (partly) proper way to inherit from Entity