Only show active layers, and sort them better

Also added a note for users that this only works in FF
No download option for GML or Shapefile (issue #818)
Merge pull request #840 from aodn/Issue_#818

No download option for GML or Shapefile (issue #818)
Merge pull request #835 from aodn/layer_filtering_improvements

Improvements for linking WMS Layers to WFS Layers