fix text message display when new text arrives

The problem was we were pulsing the display power briefly down while
reentering the ON state (because the ON states exit rule turned it off).
Instead we now just turn off the screen on entry to DARK or LS states

src/screen.h

@@ -35,6 +35,10 @@ public:
 
     /// Handle a button press
     void onPress();
+
+    /// Rebuilt our list of screens
+    void setFrames();
+private:
 };
 
 extern Screen screen;