quick frontend (app, chat): encryption handling: - new QuickApp.ENCRYPTION_HANDLERS class attribute, if True (default), encryption handlers are set - encryption plugins are retrieved on startup and cached in QuickApp.encryption_plugins list - QuickChat's encrypted boolean attribute indicate if a session is currently encrypted. This is updated automatically if ENCRYPTION_HANDLERS is set - if ENCRYPTION_HANDLERS is set, messageEncryptionStarted and messageEncryptionStopped are called when suitable.