Added a sf::Key::Unknown key code for unsupported keys

src/SFML/Window/Linux/WindowImplX11.cpp

@@ -939,7 +939,7 @@ Keyboard::Key WindowImplX11::keysymToSF(KeySym symbol)
         case XK_9 :            return Keyboard::Num9;
     }
 
-    return Keyboard::Key(0);
+    return Keyboard::Unknown;
 }
 
 } // namespace priv

src/SFML/Window/OSX/HIDInputManager.mm

@@ -1019,7 +1019,7 @@ Keyboard::Key HIDInputManager::nonLocalizedKeys(UniChar virtualKeycode)
             // This key is ' on CH-FR, ) on FR and - on US layouts.
             
             // An unknown key.
-        default:                        return sf::Keyboard::KeyCount;
+        default:                        return sf::Keyboard::Unknown;
     }
 }
 

src/SFML/Window/Win32/WindowImplWin32.cpp

@@ -820,7 +820,7 @@ Keyboard::Key WindowImplWin32::virtualKeyCodeToSF(WPARAM key, LPARAM flags)
         case '9' :           return Keyboard::Num9;
     }
 
-    return Keyboard::Key(0);
+    return Keyboard::Unknown;
 }