fdo#86579 Give uniform width to Category and Format frames

Change-Id: I55648417f56c28b0ac07ebff908805764bb2eb0c
Reviewed-on: https://gerrit.libreoffice.org/13059
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>