fix minimise button being disabled when either a parent or a dialog.

native/Avalonia.Native/src/OSX/WindowImpl.mm

@@ -63,7 +63,7 @@ HRESULT WindowImpl::Show(bool activate, bool isDialog) {
     START_COM_CALL;
 
     @autoreleasepool {
-        _isDialog = isDialog;
+        _isDialog = isDialog || _parent != nullptr;
 
         WindowBaseImpl::Show(activate, isDialog);
 
@@ -96,6 +96,8 @@ HRESULT WindowImpl::SetParent(IAvnWindow *parent) {
         auto cparent = dynamic_cast<WindowImpl *>(parent);
         
         _parent = cparent;
+
+        _isDialog = _parent != nullptr;
         
         if(_parent != nullptr && Window != nullptr){
             // If one tries to show a child window with a minimized parent window, then the parent window will be