[examples] Convert ManipulationStation example to pure python

[RussTedrake]

This was impossible when the ManipulationStation was first born, but is possible now. This would allow people to change the example for their own use cases, as intended, and avoid the temptation of cramming more into the already convoluted API.

I have a version here. Will work on migrating it soon.

cc @jwnimmer-tri in case it impacts any of your current work. (I think not)
I'm also open to feedback if anybody thinks that keeping the C++ version is important.

[jwnimmer-tri]

cc @jwnimmer-tri in case it impacts any of your current work. (I think not)

That's right. As long as we have some demo in Drake that generates image and point cloud messages so we can manually test drake-visualizer, I'm good.