EncryptEMailController: minor refactor

• Initialize 'wizard' dialog in the Private constructor, it only gets created when the Tools -> Clipboard action is triggered
• Make d->wizard a std::unique_ptr
• Rename ensuareWizardCreated() to createWizard() and make it non-const
• Close the wizard in the public class's destructor, if you close the parent app, it makes sense to close dialogs started from it
• Port to PMF signal/slot syntax
• Remove slotWizardRecipientsResolved and connect directly to recipientsResolved