Several fixes:

. make EncryptionManager::Private::terminateActivityEncryption()
asynchronous.

. add signal EncryptionManager::activityEncryptionChanged()
to inform the activity dataengine when encryption option changes for an
activity.

. fix removing encfs configuration file when disabling encryption.

. emit signals EncfsInterface::mounted() and EncfsInterface::unmounted()

. some small optimizations.