Add debug documents to SwDBManager::MergeMailFiles

Just like the debug documents in SwDBManager::MergeDocuments.

Change-Id: I275575853c80d4e19d6df5ba9d8dc689353d7c85
MM: Use SwDoc::Append for SwFeShell::Paste

Same as the SwDBManager::MergeDocuments change.

Change-Id: I44684bce187bb949dafc35a49b0aea9b46465cda
Normalize SwDBManager::Merge{MailFiles,Documents}

There is still a difference, if run mail merge throught one or
the other function (MM wizard or UNO).

Change-Id: Ia372bd3edea4c8dfb58e045fc8fed2efcb2a4fde