Schema Drag and Drop Fix

There are several bugs that I have developed a fix for. There are listed below:

1. Schema drag and drop for postgis schemas was not functional:

• Before the fix, when the user would click and drag a schema (postgis, wms, wfs, etc.) object from the catalog view to the map view, nothing would happen.
• This lack of functionality was a result of the Postgis and Postgis Experimental extension points. The existence of both extension points being instances of PostgisConnectionFactory was causing the UDIGConnectionFactory descriptor to be set to null after checking the first PostgisConnectionFactory and then checking the second PostgisConnectionFactory. Per discussion with Jesse Eichar, the Postgis Extension point was removed leaving the Postgis Experimental extension point as the only PostgisConnectionFactory element.

2. Wrong dialog prompt when dragging and dropping schema - The Add Data dialog box was appearing when dragging and dropping a schema onto the map view because of the lack of implementation for IResolveFolder instances.

3. The Project Explorer view's lowest level of display is now IMap. The layers will no longer be displayed in the context tree of the project explorer view.