MM: always copy the internal document

There is really no need to create the first document via UNO
createClone(), which is also slower then the internal copy.

Actually createClone also calls createCopy, but without
initializing the document.

Change-Id: I2d1ee6e12e37caf1b0d9bfa393fb4240516628e9
Reviewed-on: https://gerrit.libreoffice.org/10966
Reviewed-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>
Tested-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>