Add transparency to highlights

[ozgunbabur]

Consider the graph:

And this is how it looks when the edge and the target nodes are highlighted:

This is problematic because the highlighting hides the edge and node features.
Newt has an option to adjust the highlight width and color.

We would also like a setting for transparency so we can still see what's underneath. It makes sense to implement this feature into Newt and do a pull request.