Add split view for evaluation while editing impact model

[Matthias Schoettle]

When editing the impact model currently only the impact model itself can be seen. To know whether the chosen relative contribution values make sense, it would be helpful to see the evaluation of a selection right away.

Here, a split view could help, which shows the feature model on the bottom and allows to make a selection, immediately evaluation the impact model displayed above.

See also #24 (general split view).