Platform: make Configuration constructors explicit.

As it now isn't passed by pointer, this allows doing things like this:

    /* Lost all hope in this hardware */
    if(!awesomeFeatureSupported)
        createContext({});

src/Platform/AbstractXApplication.h

@@ -196,7 +196,7 @@ class AbstractXApplication::Configuration {
     Configuration& operator=(Configuration&&) = delete;
 
     public:
-        explicit Configuration();
+        /*implicit*/ Configuration();
         ~Configuration();
 
         /** @brief Window title */

src/Platform/GlutApplication.h

@@ -282,7 +282,7 @@ class GlutApplication::Configuration {
     Configuration& operator=(Configuration&&) = delete;
 
     public:
-        explicit Configuration();
+        /*implicit*/ Configuration();
         ~Configuration();
 
         /** @brief Window title */

src/Platform/NaClApplication.h

@@ -308,7 +308,7 @@ class NaClApplication::Configuration {
     Configuration& operator=(Configuration&&) = delete;
 
     public:
-        constexpr explicit Configuration(): _size(640, 480), _sampleCount(0) {}
+        constexpr /*implicit*/ Configuration(): _size(640, 480), _sampleCount(0) {}
 
         /** @brief Window size */
         Vector2i size() const { return _size; }

src/Platform/Sdl2Application.h

@@ -229,7 +229,7 @@ class Sdl2Application::Configuration {
             SDL_WINDOW_FULLSCREEN|SDL_WINDOW_HIDDEN|SDL_WINDOW_MAXIMIZED|
             SDL_WINDOW_MINIMIZED|SDL_WINDOW_INPUT_GRABBED> Flags;
 
-        explicit Configuration();
+        /*implicit*/ Configuration();
         ~Configuration();
 
         /** @brief Window title */

src/Platform/WindowlessGlxApplication.h

@@ -119,7 +119,7 @@ class WindowlessGlxApplication::Configuration {
     Configuration& operator=(Configuration&&) = delete;
 
     public:
-        explicit Configuration();
+        /*implicit*/ Configuration();
         ~Configuration();
 };