Add missing forward declaration in Message.h

Some sources cannot be compiled without the forward declaration
of struct Message.
surface: Add ifdef guard to SurfaceCS.h

Also added Doxygen comments to the header and corresponding source.
Re-add header files to VC2012 project file

The header file entries had been removed from the project for
some time, probably because of an accident.  I'm having a hard time
opening the header files from Visual Studio IDE, so they are added
again.
surface: Add GetCoverPointsCommand interpreter for gundam mod

We don't have any Entity that make use of GetCoverPointsMessage,
which in turn uses GetCoverPointsCommand, but it's used in
Gundam mod.  It provide list of cover points where mobile suits
can hide and shoot from behind.  The Airport in demo7.nut
demonstrates this new mechanism.

Previously, cover point querying message is processed by WarField,
but now we can process the equivalent EntityCommand in per-Entity
basis.