[Cmake] CMake: Adding all .txx and .h files to the DSP file

[Brad King]

> There appears to be a command called SOURCE_GROUP, but I can't get it to
> work.
> What am I doing wrong?
The SOURCE_GROUP command only places files into groups that are already   
being added to a project.  The SOURCE_FILES command only specifies .cxx   
files that need to be compiled.  Since the .txx and .h files are only
included by other cxx files, they don't show up in the project.

-Brad