Merge pull request #9326 from AvaloniaUI/fixes/osx-child-windows-cant-minimise

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

Author: maxkatz6

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

@@ -63,7 +63,7 @@
     START_COM_CALL;
 
     @autoreleasepool {
-        _isDialog = isDialog;
+        _isDialog = isDialog || _parent != nullptr;
 
         WindowBaseImpl::Show(activate, isDialog);
 
@@ -96,6 +96,8 @@
         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