Implement a return app stack

Each opened app (screen) is pushed on a stack, which is then popped from
when returning instead of hard coded "previous apps". Return swipe and
refresh directions are automatically determined from the app transition.

src/displayapp/screens/Metronome.cpp

@@ -146,7 +146,6 @@ void Metronome::OnEvent(lv_obj_t* obj, lv_event_t event) {
 bool Metronome::OnTouchEvent(TouchEvents event) {
   if (event == TouchEvents::SwipeDown && allowExit) {
     running = false;
-    return true;
   }
-  return false;
+  return true;
 }