move more file interface to std::string

Common/CommandLine.cpp

Common/Context.h

Common/CreateFile.cpp

Common/CreateFile.h

Common/GmshMessage.cpp

Common/OpenFile.cpp

Common/Options.cpp

Fltk/Main.cpp

Fltk/Solvers.cpp

Fltk/fileDialogs.cpp

Fltk/optionWindow.cpp

Geo/GModelIO_Geo.cpp

Geo/GeoStringInterface.cpp

Parser/Gmsh.tab.cpp

Parser/Gmsh.y