API documentation cleanup

[jng]

The current Web API documentation source does not cleanly build with the version of doxygen that we're using (1.8.0).

In addition, the documentation itself is missing in some parts

• Documentation of the WebApp classes (that are wrapped by the proxy classes in the the OSGeo.MapGuide.Web.dll assembly)
• Some internal classes are being leaked out. Need to compare with the exposed classes of the .net assemblies to determine which classes need to be hidden.

If you look at the generated Web API documentation for 2.4 and compare it to the "published" 2.0 version on mapguide.osgeo.org you can see there are some stark differences:

• The lack of a navigation tree
• The lack of a TOC

The build process already warns about such issues, so something is definitely amiss here.