Platform: overrideable functions in Screen can now be private.

This was done only because Doxygen wasn't able to document private
virtuals. Now it can.

src/Magnum/Platform/Screen.h

@@ -175,12 +175,13 @@ template<class Application> class BasicScreen: private Containers::LinkedListIte
             return Containers::LinkedListItem<BasicScreen<Application>, BasicScreenedApplication<Application>>::next();
         }
 
-    protected:
         /** @{ @name Screen handling */
 
+    protected:
         /** @brief Request redraw */
         virtual void redraw() { application()->redraw(); }
 
+    private:
         /**
          * @brief Focus event
          *

src/Magnum/Platform/ScreenedApplication.h

@@ -185,11 +185,7 @@ template<class Application> class BasicScreenedApplication: public Application,
            this is faster than public pure virtual destructor */
         ~BasicScreenedApplication();
 
-    #ifdef DOXYGEN_GENERATING_OUTPUT
-    protected:
-    #else
     private:
-    #endif
         /**
          * @brief Global viewport event
          *
@@ -210,7 +206,6 @@ template<class Application> class BasicScreenedApplication: public Application,
          */
         virtual void globalDrawEvent() = 0;
 
-    private:
         #ifndef DOXYGEN_GENERATING_OUTPUT /* https://bugzilla.gnome.org/show_bug.cgi?id=776986 */
         friend Containers::LinkedList<BasicScreen<Application>>;
         friend Containers::LinkedListItem<BasicScreen<Application>, BasicScreenedApplication<Application>>;