Timer App : add background label to ensure that the app will be displayed correctly after a full refresh (HW scrolling transition).

Code cleaning and rename methods.

src/displayapp/DisplayApp.cpp

@@ -207,7 +207,7 @@ void DisplayApp::Refresh() {
       case Messages::TimerDone:
         if (currentApp == Apps::Timer) {
           auto* timer = static_cast<Screens::Timer*>(currentScreen.get());
-          timer->setDone();
+          timer->SetDone();
         } else {
           LoadApp(Apps::Timer, DisplayApp::FullRefreshDirections::Down);
         }