Add Stopwatch application

It can be started, stopped, and cleared. It also continues to run if
you leave the screen.

firmware/Application/ScreenManager.h

@@ -62,6 +62,7 @@ private:
 
     std::array<Screen *, MAX_SCREEN_STACK> m_screen_stack;
     std::size_t m_screen_stack_depth;
+    bool m_root_initialized;
 
     BSP::DisplayDriver &m_display;
     BSP::ButtonManager &m_buttons;