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:

* [x] minor simplification in `OrthogonalPath`
* [x] drop dependency on sklearn by including the dataset in vispy (#389).
* [x] ~~support for n-dimensional data points (instead of 4 dimensions max here), related to #390~~ (to be done later)

Close #389.

On the other hand, I seem to remember that it was a design decision not to do any conversion in gloo...  @almarklein @rougier what's the best thing to do here (and in the other examples and in the documentation...) ?