[OSX] allow child windows to be shown on top of fullscreen windows.

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

@@ -206,7 +206,10 @@ public:
                 auto window = Window;
                 Window = nullptr;
                 
+                try{
                 [window close];
+                }
+                catch(NSException*){}
             }
             
             return S_OK;
@@ -724,6 +727,7 @@ private:
             if (cparent->WindowState() == Minimized)
                 cparent->SetWindowState(Normal);
             
+            [Window setCollectionBehavior:NSWindowCollectionBehaviorFullScreenAuxiliary];
             [cparent->Window addChildWindow:Window ordered:NSWindowAbove];
             
             UpdateStyle();