Improve layer visibility/purpose indication

[mdehoog]

When toggling many layers, there is no visual indication of what happened. This is often because the layer is out of zoom range (lots of empty top levels), or the view is outside of the layer's sector. Also, toggling elevation model layers doesn't seem to do anything unless vertical exaggeration is set very high.

Need to add a visual indication to the layer node in the tree about whether the layer is visible or not, or an elevation model or not. Also, the zoom-to-layer handler should know about empty levels, and either zoom to a level at which the layer is visible, or alert the user somehow.