Editors: Get rid of keeping separate lists of "duplicated" editors.

And use documents for the list of currently open documents, instead of
using a set of "original" editors that needed to be separately managed.

Change-Id: I3379ca44f03646399d3ecb0e775d7f1fd81a6bcf
Reviewed-by: Eike Ziller <eike.ziller@digia.com>