This saves the loading time for large document. Use it if you
want to present the generated document to the user or write a
mail merge unit tests.

This includes:
  * renaming "only" to "shell" at multiple occurences
  * dropping the DBMGR_MERGE_SINGLE_FILE special type for the mail
    merge wizard in favour of an additional SwMergeDescriptor
    boolean
  * cleanup and renaming of the internal merge type enums, so these
    actually  match the diffferent merge targets: printer, email,
    file and shell

Change-Id: I33c6773972195193687ba9c3e12b562310d330c1