Refactored IDocumentLinksAdministration out of SwDoc.

To the new class DocumentLinksAdministrationManager.
Additional to the Interface methods SwDoc::SelectServerObj was also moved
and sw/source/core/doc/docdde.cxx was deleted as it became empty.
Also fixed OUString usage in IDocumentLinksAdministration.hxx .

Change-Id: I1f2bf0881a7d4add9c657b6441851ae14ad8d161