press '0' to exit the Tutorial

quit.cpp

@@ -427,6 +427,7 @@ void showMission() {
       dialog::addItem(XLAT("next slide"), SDLK_RETURN);
       dialog::addItem(XLAT("previous slide"), SDLK_BACKSPACE);
       dialog::addItem(XLAT("list of slides"), '9');
+      dialog::addItem(XLAT("exit the Tutorial"), '0');
       }
     else
       dialog::addBreak(200);

tour.cpp

@@ -248,6 +248,11 @@ bool handleKeyTour(int sym, int uni) {
     pushScreen(ss::showMenu);
     return true;
     }
+  if(NUMBERKEY == '0') {
+    tour::start();
+    pushScreen(showStartMenu);
+    return true;
+    }
   return false;
   }