EncryptEMailController: minor refactor

• Initialize 'wizard' dialog in the Private constructor, it gets created on demand by the contoller that's created when the Tools -> Clipboard action is triggered
• Rename ensuareWizardCreated() to createWizard() and make it non-const
• Make wizard a built-in pointer, QPointer serves no purpose now as the dialog is always created in the Private constructor
• Port to PMF signal/slot syntax
• Remove slotWizardRecipientsResolved and connect directly to recipientsResolved
• Replace one QTimer::singleShot() with queued method invocation