Improve HIG in OpenCL Add/Edit dialog

Change-Id: I8798e9c54aa85b75eda729d2aa8fbea656712894
Reviewed-on: https://gerrit.libreoffice.org/13584
Reviewed-by: Tor Lillqvist <tml@collabora.com>
Tested-by: Tor Lillqvist <tml@collabora.com>