OpenEditorsModel API: Use 'document' more where is about documents.

Rename OpenEditorsModel to DocumentModel.
In the DocumentModel also make the distinction between "restored"
document (i.e. just info about file name, display name, id), "opened
document" (i.e. document with IEditor and IDocument), and "document"
(which refers to any).

Change-Id: I01ebe10ec84aab5fe81e54be6bec14f653f28771
Reviewed-by: Eike Ziller <eike.ziller@digia.com>