Detect grab potential on mouse move and change the cursor

[rafaqz]

We should be able to show with the cursor if you can grab a line or vertex when you move over them with the mouse.

This should reduce a lot of confusion over what will happen when you click in a point.

[asinghvi17]

If you can't change the cursor in a backend-independent way, you can at least change the color of the line or vertex...

[rafaqz]

Maybe just the vertex scatter point border color would be enough

[asinghvi17]

That sounds good to me