Wallet Transactions Export: Add BIP-329 support

[dennisreimann]

Adds support for the BIP 329 export format.

Sample transactions export:

{"type":"tx","ref":"9eb4b1f69dc0aad88f2530fdd124c77021c1633500e14369d0896bf75a025596","label":"microtransaction"}
{"type":"tx","ref":"98ee2e20e45967eeda330b191d881584d1e02ea05e633e4cb7c48aea8980fc63","label":"test"}
{"type":"tx","ref":"84ce8b198d85e715588da114b7b441833cfc4892ba9aa1f7f3cfbfdcf2ca1235","label":"Mining Income, microtransaction, test"}

Currently only the transaction data is exported, but I think we can also leverage the new labeling system to export other types, like addr as well. @NicolasDorier

Closes #4536.

[NicolasDorier]

Can I test it somewhere with another wallet? I believe you should also export the addresses. Me or @Kukks can add that to the PR after.

Also it should be Wallet Labels Export (BIP-329) rather than underscriptive JSON (bip-329) I believe.

[NicolasDorier]

I give up implementing address export for now.
I need to refactor the export stuff so we don't need another hack overload.