don't open separate browser window on iframe load

https://anki.tenderapp.com/discussions/beta-testing/627-anki-210-beta-1#comment_43008395

aqt/webview.py

@@ -56,6 +56,8 @@ class AnkiWebPage(QWebEnginePage):
              dict(a=line, b=msg+"\n")))
 
     def acceptNavigationRequest(self, url, navType, isMainFrame):
+        if not isMainFrame:
+            return True
         # load all other links in browser
         openLink(url)
         return False