Add rounded corners for rect/quad glyphs

[mrocklin]

It would be nice to have a glyph that was like rect/quad, but that supported rounded corners. This would make using Bokeh for diagramatic workflows slightly smoother/slicker.

[mattpap]

I've been wanting to implement this for a long time, so tentatively adding to 2.4.

[mrocklin]

I'm happy to provide an excuse :)

[Pierretherenard]

Does that mean 'legend' and 'label' will be able to have round corner as well?