Refreshable Layers

WW is missing a very powerful feature found in Google Earth - layers that get loaded over the network. So layer definitions no longer have to be static and can be generated by PHP and other server side methods. This brings up the realization that Add Ons are the really important thing to make sure we can load, since they can contain point and line info as well (a poor mans GML) Also Shapefile xml and the associated .shp, .dbf and other files are also a candidate for loading from URL's as well.

It would enable add ons like Plane Tracker and Earthquake to be entirely server side affairs.

Issues

 * 1) Add-ons - Right now Add-on's are treated kind-of like layers. Loading them should be as easy as loading a normal layer, their complication is that they have local resources they need to access (icons). We could make the system able to load zipped XML files (xmz) so an add-on can have all of its resources carried along with it. Then this system could also be used to make file system Add-ons self contained as well.
 * 2) Shapefiles - If we do the .zip file solution for Add-ons we can have self-contained shapefiles and we can load Shapefile .xml configs just like layers.
 * 3) Altova - Currently we cannot easily alter the schema a generate the backend code that parses the XML files in /Config to make objects for use in WW. Note: Chris is in the process of removing this dependancy (as of 1-13-06)
 * 4) Blocking - need to spin off a thread to load network resources in case a server is slow/down. There is an Async App. Block to do this already.
 * 5) Clickable URLs - Do we add a worldwind://layer/<> schema to the system or use an extension (.wml)?
 * 6) Persistance - currently in GE there are 'Temporary Places' and 'My Places', before exiting you are prompted to add the temp item into your persistant store, what do we want to do for clicked on URL's that add a layer into LM.
 * 7) Metaconfig Files - Why cant we have all of a /Config directories contents be totally loaded from a URL? This would allow large organizations to push layers out to their users that might be generated from other tools (ie. ArcMap/IMS or MapServer) or are new - RSS Layers. (I realize that this is partially solved by the fact that LayerSets can contain multiple ChildLayers which could all be URL loaded)