Platform: minor cleanup.

src/Magnum/Platform/AbstractXApplication.h

@@ -43,11 +43,7 @@
 #include "Magnum/Math/Vector2.h"
 #include "Magnum/Platform/Platform.h"
 
-namespace Magnum {
-
-class Context;
-
-namespace Platform {
+namespace Magnum { namespace Platform {
 
 namespace Implementation {
     template<class, class, class, class> class AbstractContextHandler;

src/Magnum/Platform/GlutApplication.h

@@ -39,11 +39,7 @@
 #include "Magnum/OpenGL.h"
 #include <GL/freeglut.h>
 
-namespace Magnum {
-
-class Context;
-
-namespace Platform {
+namespace Magnum { namespace Platform {
 
 /** @nosubgrouping
 @brief GLUT application

src/Magnum/Platform/Sdl2Application.h

@@ -42,11 +42,7 @@
 #include <SDL.h>
 #include <SDL_scancode.h>
 
-namespace Magnum {
-
-class Context;
-
-namespace Platform {
+namespace Magnum { namespace Platform {
 
 /** @nosubgrouping
 @brief SDL2 application