[CMake] add files to visual studio without compliation

[Peter Vanroose]

> (1) How can I add files to a project/workspace without creating a compilation 
> target for them using cmake ?

By making sure that the file extension is not .cxx or .cpp but e.g. .txx or .h
Typically, .txx is used for "template source files", i.e., implementations 
for templated functions, which are included either directly by the header 
file containing the class declaration, or by an instantiation .cxx file.


--	Peter.