Support for distributed rootPath management

Now it's possible to distribute add_project_file()
calls across multiple subdirectories.
This should support modular branching better, and
is more compatible with a CMake-style build layout.

Also cleaned up class RootPaths handling.
* Eliminated "_rel" suffixed variables, and instead
  created a separate rootPathsRel object.
* Removed kludgy member function from class RootPaths,
  and changed everything to global functions.