Layertree for GeoRSS/KML resources

[armin11]

Generate a layertree to manage the GeoRSS/KML resources in the mapbender client. The layertree is much more easier to handle than the actual tablesorter alone. Maybe it is a quit good idea to have both possibilities:

1. Access the geoobject as single leafs over the tree structure
2. Access the tablesorter over a simple tree, where only the resources are leafs

This maybe be steered by an element var.