move MergeFile & co interfaces to std::string

Common/CommandLine.cpp

Common/Context.h

Common/CreateFile.cpp

Common/CreateFile.h

Common/Gmsh.cpp

Common/Main.cpp

Common/OpenFile.cpp

Common/OpenFile.h

Common/Options.cpp

Fltk/GUI.cpp

Fltk/GUI.h

Fltk/Main.cpp

Fltk/Makefile

Fltk/contextWindow.cpp

Fltk/extraDialogs.cpp

Fltk/fieldWindow.cpp

Fltk/menuWindow.cpp

Fltk/solverWindow.cpp

Fltk/visibilityWindow.cpp

Geo/GModel.cpp