HIG fixes for Document Properties dialog

* Add missing colons to labels that precede controls
* Standardize Help button’s position
* Better label for Add Property button
* Add a frame for the lonely Embed Fonts option

Change-Id: If12708161f2345e66ab3f49fd744bcad00d4ecc6
Reviewed-on: https://gerrit.libreoffice.org/12096
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>