On copying slides from another presentation into one, we end up
with two XBitmapItems with the same name and different properties.

because the names are supposed to be unique, only one gets exported
so the presentation looses graphics on export

there is a bunch of places which use checkForUniqueItem to block
this from happening, add another at the point where we insert
the new page into the model

Change-Id: If708b1997c7e83e4787d9d54f84c8b0516e6cf6b
Reviewed-on: https://gerrit.libreoffice.org/30664Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>