Support encryption of local cache

On some company, by their own security policy, privacy data like addressbooks need to be encrypted to guard from data stolen. By default the connection between CalDAV servers are encrypted via TLS/SSL, but local data stored in the lindexedDB are not. So, this merge request inserts encryption/decryption operations while writing/reading data of the indexedDB. I'm happy if this feature is supported by the upstream.

The secret key is automatically generated on the initial startup, and stored to the preferences DB. So, please note that the encryption is still weak for attacks like stolen of whole profile data or accessing to the local file system. This change just guards data from stolen per file (thus, impossible to read data in the SQLite DB). How do you think about that?