Fix split editor area pane focus

If an editor contains tabs, after certain operations (e.g., navigating
the editor tabs using keyboard shortcuts) a change in focus
sets the editor area pane `active_tabwidget` to one of those internal tabs,
rather than to the tab of the editor in the editor area.

This causes problems, e.g. in this example I added two editors, and one got inserted inside the other:

![image](https://f.cloud.github.com/assets/314633/682445/8c23792e-d9c7-11e2-8e49-1fd64085dc1e.png)