Fix closing and restoring editors in presence of temporary editors.

* Editor manager should abort closing core, if there's a ICoreListener
that aborts closing of a editor.
* Don't save state of temporary editors, and don't save them as open in
  a splitter. So restoring the editor manager state won't try to reopen
  temporary editors.

Task-number: QTCREATORBUG-2845
Task-number: QTCREATORBUG-2841
Reviewed-by: Tobias Hunger