Better labels for extension manager

Also align the heading on the left ("Display Extensions")

Change-Id: Ied410032d9212ea2540be01447b175e0122e5707
Reviewed-on: https://gerrit.libreoffice.org/10524
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>