QMacStyle: Configure checkbox NSButton as tri-state

Qt allows this as a property of QCheckBox, so it should
be enabled in its Cocoa counterpart.

Change-Id: I88eb6dddabb173050c4fe7229f15c768181ef527
Task-number: QTBUG-69453
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

src/plugins/styles/mac/qmacstyle_mac.mm

@@ -1837,6 +1837,8 @@ NSView *QMacStylePrivate::cocoaControl(CocoaControl widget) const
         auto *button = static_cast<NSButton *>(bv);
         button.buttonType = buttonType;
         button.bezelStyle = bezelStyle;
+        if (widget.type == Button_CheckBox)
+            button.allowsMixedState = YES;
     }
 
     return bv;