Simplify and beautify the Similarity Search dialog

* Remove unnecessary containers and unhelpful frame
* Center main container
* Right-align labels to remove large gaps between controls
* A bit of separation for the Help button

This should serve as a model for similar dialogs.

Change-Id: I1964e8909159506484950df9d8ef7283809cce5f
Reviewed-on: https://gerrit.libreoffice.org/13057
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>