ExtraForce trait and other useful types aren't exposed publicly

[vivlim]

The ExtraForce trait seems to be necessary to implement extra composable forces, but it isn't exposed publicly.

Also there are other types and helpers that would've been helpful for implementing custom node/edge displays. I was able to do that by copying the default implementations and all the private helpers they depend on. It would be nice if some of those were public, especially EdgeShapeBuilder and EdgeShapeProps.

[steadfastnz]

and the members of SettingsStyle, etc.