NewSignEncryptFilesWizard: fix implicit state machine

• propagate information about forcing signing/encryption to OperationPage so it can properly enable/disable the controls
• remove some asserts that don't hold in transient states
• fix startId() to take the archiving options into account
• fix a buglet in setCreateArchiveUserMutable()

Also:

• Complain when the user selects archive+sign-only, which isn't yet implemented.

svn path=/branches/kdepim/enterprise4/kdepim/; revision=1076646