multi: Add decred.

[JoeGruffins]

I'm looking into the possibility of adding decred to cake wallet. Would this be supported by the cake wallet maintainers? I can do all of the work of adding but require review and merging ofc. Will add to this issue with a more fleshed out plan if the endeavor will be supported.

[SamsungGalaxyPlayer]

Hi Joe, please email justin@cakewallet.com and we can chat about it later this week.

[JoeGruffins]

1322 Adds decred. This is meant to add everything we need. There are still a few things we want to update after this.

TODO list:

• [ ] Make the UI around persistent peers better. It looks like you can set multiple but currently it will only look at the last set I guess. We don't need one set at all but cake seems to barf if you have none, not saving the wallet at all. So it's currently just a port if we are letting the libwallet find peers for us.
• [ ] We need to add address discover as a sync topic because this can take a while. Should add fetching headers and rescanning as well.
• [ ] We don't want to give out an address before sync is complete, so need to disable the receive screen or something until synced. Currently it returns nothing and shows the default screen.
• [ ] We don't want the user to go to the send screen when using a watching only wallet.
• [ ] There needs to be a way to tell that one is using a watching only wallet rather than a "normal" one.
• [x] The next wallet version will have the ability to sync from a birthday, so the initial sync time will be greatly reduced. We will want to wait for a dcrwallet release so it may be a bit before we can use it here though.
• [x] Related to the above but rescans should just start from the wallet birthday. The current date -> block method is temporary and not precise.
• [ ] Test buy, sell, and exchange on mainnet.
• [ ] Users should be warned about data usage if initial sync would happen without wifi. They should have the option to pause sync if not using wifi.
• [ ] Implement restore from hardware and qr code.
• [ ] DCR wallet address qr has no usable data but it should (btc wallet address qr has a trustwallet link)