Refactor highlight/select features in both feature grids

[hwbllmnn]

Both grids currently use setStyle on the original layer in order to highlight and select features. That's an unclean solution which may cause lots of trouble. Instead, separate temporary layers should be used instead.

[dnlkoch]

Note: Currently the selection of features is broken in the FeatureGrid.

[marcjansen]

What is the current state here?

[dnlkoch]

Selection bug seems to be fixed, but the usage of setStyle is still present and should be fixed.