[gtkmm] Glib::build_filename() convenience function

[Andreas Holzmann <Andreas Holzmann epost de>]

Hi,

in glib (c) you can write something like
  g_build_filename(dir, file, NULL) 
to append a file to a directory path.

In glibmm (c++) you have to write
  std::vector<std::string> path;
  path.push_back(dir);
  path.push_back(file);
  Glib::build_filename(path)

So, how about adding the following convenience function:
  Glib::build_filename(const std::string& elem1, const std::string& elem2)

Comments? 
If it's OK I'll commit this change.

 Andreas