OMEMO support

Implementation of XEP-0384 v0.3.

Depends on libsignal-protocol-js at runtime.

To disable OMEMO at build time (and thus remove the libsignal dependency) compile with the NO_OMEMO flag.

TODO / known issues

• No caching of remote contact devices
• No API to control encryption of outgoing messages
• No API to determine cryptographic identity of message sender
• Persistence: only IndexedDB backend is currently implemented
• Encryption status reported by the API can be forged by sender
• Outgoing messages are not encrypted to the sending account's other devices
• No support for group chats