Add tooltips for non-obvious, unlabeled buttons in redlinefilterpage.ui

Change-Id: I9a39f6eaa3555e32a64f6c2c479685d98c4087f8
Reviewed-on: https://gerrit.libreoffice.org/11670
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>