QmlDesigner: Bind meta info cache to model

A global variable is not helping for multithreading and the cache has
anyway a 1:1 relationshit to the model.

Change-Id: Ic05f32744b70ef63aa5ae7d475afc0b5b17f2f09
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/qmldesigner/components/integration/designdocument.cpp

@@ -579,7 +579,7 @@ void DesignDocument::paste()
             }
             view.setSelectedModelNodes({pastedNode});
         });
-        NodeMetaInfo::clearCache();
+        view.model()->clearMetaInfoCache();
     }
 }