decred: Add used address history.

[JoeGruffins]

Part of #1938

This adds an address history. Currently very simple but can be built upon if necessary.

It changes the sync check back to 5 seconds while syncing only. I feel like 30 seconds is rather long in the beginning. Hope this is alright.

It reverts wisdom's pubspec changes, they tell me the downgrade was a problem with their flutter version after all. Also added testnet by them which can be used for testing, I can provide testnet funds please message.

There were some formatting errors apparently, although I thought I had been updating those.

[JoeGruffins]

I guess the formatting problem is on our side. I am using dart format but I guess this is not correct, what command should I use?

Also, there is currently a problem with wallet groups. Because of putting the birthday in the mnemonic, we are unfortunately not compatible with other wallets atm...

[JoeGruffins]

Added another commit. We do use the bip39 word list, which is what I think this is about.

[JoeGruffins]

It looks like we have a bug in wallet upstream that has been fixed but may cause sync to stop at the very end. I will update the dcrwallet dep to include this. relevant pr decred/dcrwallet#2445

[JoeGruffins]

Now formatted with 100 width lines.

[JoeGruffins]

Code updates suggested by @itswisdomagain https://github.com/cake-tech/cake_wallet/compare/a7266c24a573429b5d4df1c3f216e0dfcc4e5367..1fb22ab832d390db176ad5b1db02e7c6785c0031

I can't figure out how to get the address you tap on in the list to become the one the screen is showing.