New n-dimensional scatter plot example with GPU-based smooth projections

Not ready for merge, but you can already try it!

Things to be done:

* [ ] minor simplification in `OrthogonalPath`
* [ ] support for n-dimensional data points (instead of 4 dimensions max here), related to #390
* [x] drop dependency on sklearn by including the dataset in vispy (#389).

Close #389.

Done.