Generate key events for caps lock when the modifier state changes

This is not accurate because modifier state and key state are not the
same thing for caps lock, but at least some events are generated
instead of nothing.

src/SFML/Window/OSX/SFKeyboardModifiersHelper.mm

@@ -69,6 +69,7 @@ struct ModifiersState
     BOOL leftAlternateWasDown;
     BOOL leftControlWasDown;
     BOOL rightControlWasDown;
+    BOOL capsLockWasOn;
 };
 
 
@@ -140,6 +141,7 @@ void initialiseKeyboardHelper(void)
     state.rightAlternateWasDown   = isKeyMaskActive(modifiers, NSRightAlternateKeyMask);
     state.leftControlWasDown      = isKeyMaskActive(modifiers, NSLeftControlKeyMask);
     state.rightControlWasDown     = isKeyMaskActive(modifiers, NSRightControlKeyMask);
+    state.capsLockWasOn           = isKeyMaskActive(modifiers, NSEventModifierFlagCapsLock);
 
     isStateInitialized = YES;
 }
@@ -202,6 +204,16 @@ void handleModifiersChanged(NSUInteger modifiers, sf::priv::WindowImplCocoa& req
         sf::Keyboard::Scan::LControl, sf::Keyboard::Scan::RControl,
         requester
     );
+
+    // Handle caps lock
+    processOneModifier(
+        modifiers,
+        NSEventModifierFlagCapsLock,
+        state.capsLockWasOn,
+        sf::Keyboard::Unknown,
+        sf::Keyboard::Scan::CapsLock,
+        requester
+    );
 }