Platform: make Screen::{focus,blur}Event() optional to implement.

src/Magnum/Platform/Screen.h

@@ -283,7 +283,7 @@ template<class Application> class BasicScreen:
          * Called when screen is focused using @ref BasicScreenedApplication::focusScreen()
          * or @ref BasicScreenedApplication::addScreen().
          */
-        virtual void focusEvent() = 0;
+        virtual void focusEvent();
 
         /**
          * @brief Blur event
@@ -292,7 +292,7 @@ template<class Application> class BasicScreen:
          * @ref BasicScreenedApplication::addScreen() or before the screen is
          * removed from application using @ref BasicScreenedApplication::removeScreen().
          */
-        virtual void blurEvent() = 0;
+        virtual void blurEvent();
 
         /**
          * @brief Viewport event

src/Magnum/Platform/ScreenedApplication.hpp

@@ -108,6 +108,9 @@ true>::textEditingEvent(TextEditingEvent&) {}
 template<class Application> BasicScreen<Application>::BasicScreen() = default;
 template<class Application> BasicScreen<Application>::~BasicScreen() = default;
 
+template<class Application> void BasicScreen<Application>::focusEvent() {}
+template<class Application> void BasicScreen<Application>::blurEvent() {}
+
 template<class Application> void BasicScreen<Application>::viewportEvent(ViewportEvent& event) {
     #ifdef MAGNUM_BUILD_DEPRECATED
     CORRADE_IGNORE_DEPRECATED_PUSH