Add Opacity Encrypted Messaging to Opalcoin

 
# Add Opacity Encrypted Messaging to OpalCoin

Adds encrypted messaging to opalcoin.

### Changes
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* Adds rpc and qt implementations of encrypted messaging to any opalcoin address
* Adds tab for Encrypted Messaging in Qt Wallet
* Added  lz4 and xxhash libraries
* Added mrichtexteditor plugin

### Issues
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* Requires manual scan of blockchain to add EM, we should look into fixing this in the future