Custom Layout mechanism

[blitzarx1]

I think it will be relevant to have a possibility for the user providing custom layout for the widget which will compute node position on every frame from node and graph state.

Several points should be noted:

1. Layout and drag conflicts.
2. Layout and user set location conflicts.
3. Layout is a stateful implementation, which keeps state for example in Graph wrapper.
4. There also should be provided a default implementation of the Layout trait.
5. Passing layout can be done via constructor methods or pure generics approach as it is done with NodeDisplay and EdgeDisplay.

To showcase this new feature configurable example can be used where forces simulation logic and node positions computation will become implementation of the Layout trait.