Replce Fl_String in Fluid with std::string.

documentation/src/common.dox

fluid/ExternalCodeEditor_UNIX.h

fluid/Fd_Snap_Action.cxx

fluid/Fd_Snap_Action.h

fluid/autodoc.cxx

fluid/autodoc.h

fluid/code.cxx

fluid/code.h

fluid/codeview_panel.cxx

fluid/codeview_panel.fl

fluid/fluid.cxx

fluid/fluid.h

fluid/fluid_filename.cxx

fluid/fluid_filename.h

fluid/mergeback.cxx

fluid/mergeback.h

fluid/settings_panel.cxx

fluid/settings_panel.fl

fluid/shell_command.cxx

fluid/shell_command.h