Tiny outline for handles + off-curve nodes

[ollimeier]

From @GaetanBaehr:

with a grey background image, I don't really see my offcurve on the left, and I actually don't see at all the one on the right, looks like it's behind the image or something

Note from @justvanrossum:

Yeah, the on-curve regular nodes have a tiny outline, we should probably do the same with handles + off-curve nodes.

[justvanrossum]

I think the "underlay" outline is only done for selected on-curve nodes. Perhaps this should be done for everything.