Node manager

[ben-kaufman]

• Add node and node manager classes
• Allow adding, removing, and switching between nodes
• Fix internal node functionality
• Remove old Bitcoin Core settings screen
• Migrate existing node configurations to the new format
• Fix the tests
• (Optional, maybe in another PR) Make wallets belong to a specific node (this could be a problem currently if user has multiple nodes of the same network)
• (Optional, maybe in another PR), allow multiple "built in" nodes for multiple networks.

[k9ert]

Looks good to me and works pretty much as expected.
The Logging button "see bitcoind logs" for the internal node doesn't point to the new directory, though.

[k9ert]

There is imho one more question: How do we migrate the data on the user-side already using internal nodes? There are two possibilities:

• Do it automatically somehow. In that case we should try to get a more generic mechanism in place which executes specific funtions based on specific configurations (like an array of executed migration-functions).
• Or we simply don't do that and let the people recreate their internal node and this needs to be documented doemhow.

What do you think? I tend to the second solution. The first one is pretty huge and should be a PR of itself. And maybe we want to still defer that mechanism until we might change persistence-technology.

[ben-kaufman]

This should already work with migrating an existing internal node, I also tested and it worked well. The code responsible for this is at https://github.com/cryptoadvance/specter-desktop/pull/1146/files#diff-88c6a6cabbeff77aafe908b914ca9b08c2c0f5942b54b10b782fbc9dded8c6a6R569

[k9ert]

There are known issues with the tor-test in cirrus, that's why this test is failing. Locally, all tests are green. That's why i'll merge this now nevertheless.