Make it easier to interact with nodes without fill

[magjac]

Currently, unfilled nodes must be interacted with through the perimeter or through the node text which makes it a little harder to select them or to draw edges from and to them. This is analogous to how the URL attribute works:

For svg [...] output, the active area for a node is its visible image. For example, an unfilled node with no drawn boundary will only be active on its label.

Filled nodes can be interacted with anywhere on or inside its perimeter, including its background.

Ideas

• When displaying the graph, set fill="transparent" on the SVG basic shape.