Add basic multi-threading for each mimetype

Change-Id: Ife0766ddd259bb7d86a9c7bdcf3e9c2849208cf0
Reviewed-on: https://gerrit.libreoffice.org/7123
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>