Make it easier to build wms layer trees

At FOSS4G some users lamented that the layer grouping functionality hides the underlying layers that some clients can use to generate a tree like legend (MapFish I believe). We actually can use the wmspath attribute to build layer trees, but it's messy and allows a layer to stay only within one tree.
With the new UI we should probably merge the two concepts and allow either to build an opaque layer group, or a group that's indeed a visiable tree (eventually allowing the same layer to be visible multiple times in different trees).