fdo#51525 Add Paste Unformatted Text to Impress.

Change-Id: I4eff66ee2990fd7bf6a9be6b9f0801e7a70206e9
Reviewed-on: https://gerrit.libreoffice.org/7783
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>