Fix closing editor with ctrl+w or "x" or File > Close

If there are other editors on the document visible in some split, it
should not close the document, but only the editor.

Task-number: QTCREATORBUG-9346
Change-Id: Idce1ae2f518d4c6e875d86f9831d41c46c06361c
Reviewed-by: Eike Ziller <eike.ziller@digia.com>