MM: introduce SwDoc::Append helper function

This drops all the specialized, workaround code from MM, introduced
to use the SwFEShell::Paste function and additionally merges
and renames SwDoc::Paste into SwDoc::Append.

There is still a little common codepath, therefore this adds
comments to always update both functions.

Change-Id: I704b3ef3257dd977dac95e16e25049ff8ade97ed
Reviewed-on: https://gerrit.libreoffice.org/10967
Reviewed-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>
Tested-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>