Refactor logic + cleanup + bug fixes

[ben-kaufman]

• Breaks down logic.py into a file for each of its classes.
• Remove the wallet and device managers iterability, replace it with a regular dict property.
• Some code cleanup and refactoring.
• Add more testing for device_manager, device, and wallet_manager (more Specter and Wallet tests soon hopefully).
• Fix bug when saving changes for settings (https://github.com/cryptoadvance/specter-desktop/pull/160/files#diff-f3f35519e1706a65feabaffbf0076002L257).
• Fix Bitcoin Core 0.20.0 address labeling compatibility issue.

[stepansnigirev]

When broadcasting the transaction I get an error:
'Specter' object has no attribute 'wallets' in controller.py line 70.

Otherwise ACK :)

[ben-kaufman]

@stepansnigirev thanks for testing, I've encountered it while continuing the refactoring too, I think now it should be fine.
Not sure if we should merge this already so it doesn't get too big, or wait to complete the refactoring of wallets and devices here as a single PR, so whatever you think is better.

[stepansnigirev]

I think let's continue refactoring here. It would be nice to finish splitting device and wallet classes in this PR.

[k9ert]

LGTM so far. I was a bit concerned that renaming logic.py back to specter.py (which caused trouble in the past) would again cause trouble, but that doesn't seem to be the case.
But i guess we agree that this needs to be backwards-compatible for v0.19.1, right?!

[stepansnigirev]

Invalid sorting of the pubkeys...

[stepansnigirev]

A few changes for managing sortedmulti - by deafult we should try to use sortedmulti, if we fail - then we run a per-address addition.
The changes also batch all requests into a single rpc call.