Adjust WebEngineEleement _js_dict in remove_blank_target

Otherwise, when checking later if the element has target=_blank set, we still
think it is.

This is not perfect as remove_blank_target also removes it from sub-elements,
but it's a good start.

qutebrowser/browser/webengine/webengineelem.py

@@ -147,6 +147,8 @@ class WebEngineElement(webelem.AbstractWebElement):
         return QRect()
 
     def remove_blank_target(self):
+        if self._js_dict['attributes'].get('target') == '_blank':
+            self._js_dict['attributes']['target'] = '_top'
         js_code = javascript.assemble('webelem', 'remove_blank_target',
             self._id)
         self._tab.run_js_async(js_code)