create a master document template type

of application/vnd.oasis.opendocument.text-master-template with
suffic otm

https://lists.oasis-open.org/archives/office-comment/201002/msg00042.html

desktop/icons/oasis-master-document-template.icns is just a copy
of
desktop/icons/oasis-master-document.icns
because I can't draw

Change-Id: I0d18c79c4c893e97505052884ee8be97d0f117a1
Reviewed-on: https://gerrit.libreoffice.org/11350
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>