Expert config: Introduce configuration key to disable expert config.

It is /org.openoffice.Office.Common/Security/EnableExpertConfiguration with
values true/false; defaults to true.

Change-Id: I5a18d16182c28a13169677b11f68884f34c11c63
Expert config: 6px between lines + center the button.

Change-Id: I91899493479eb60c955dac3891a38913ad871376