move change directory function to interpreter class * interpreter.h, interpreter.cc (interpreter::chdir, Vlast_chdir_time): Move here from change_directory in dirfns.cc. Change all uses. * main-window.cc (main_window::set_current_working_directory): Use interpreter function directly instead of going through DEFUN function. * dirfns.h: Delete. This file no longer declares any functions or variables. Remove remaining include statements from other files. * libinterp/corefcn/module.mk: Update.

--- a/libgui/src/main-window.cc	Thu Sep 12 16:55:53 2019 +0200
+++ b/libgui/src/main-window.cc	Fri Sep 13 16:22:34 2019 -0400
@@ -989,11 +989,11 @@
     if (fileInfo.exists () && fileInfo.isDir ())
       {
         emit interpreter_event
-          ([xdir] (void)
+          ([xdir] (interpreter& interp)
            {
              // INTERPRETER THREAD
 
-             Fcd (ovl (xdir.toStdString ()));
+             interp.chdir (xdir.toStdString ());
            });
       }
   }