RFC: Migration from NSUserNotificationCenter to UNUserNotificationCenter on macOS

[hebasto]

Apple suggests to use UNUserNotificationCenter instead of deprecated NSUserNotificationCenter.

I've played (on macOS 10.15) a bit with UNUserNotificationCenter and always getting the authorization error "Notifications are not allowed for this application".

Not sure, but it seems the root of this error is unsigned/unnotarized application.

[hebasto]

After a discussion in bitcoin/bitcoin#29362, this issue has come to our attention again.

In response to concerns raised in #114, it appears that migrating to the new UNUserNotificationCenter remains the only viable option. Therefore, I conducted further research and discovered the following:

The UNUserNotificationCenter requires two conditions to be met in order to be granted permissions from the OS:

1. The software must be bundled. **
2. The software must be signed.

This means that running ./src/qt/bitcoin-qt will not generate any notifications. Users must bundle it using make deploy, self-sign it with codesign -s - ./Bitcoin-Qt.app, and then run it using open ./Bitcoin-Qt.app.

If these limitations are acceptable for macOS users and developers, I am ready to continue working in this direction.

** Notes:

1. Our current hack with the substitution of the bundleIdentifier method does not help.
2. https://developer.apple.com/forums/thread/679326
3. https://forums.developer.apple.com/forums/thread/724249

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cc @Sjors @fanquake @theuni @achow101 @luke-jr @johnsBeharry

[Sjors]

I think (2) is fine. We already sign the release build, and anyone who can compile from source can run the extra codesign command (maybe as part of make deploy?).

If (1) just means that they have to use make deploy in order to get notifications working, that seems fine.

[alfonsoromanz]

I was not familiar with the process of bundling and signing the application, however I ran the commands mentioned (make deploy, codesign -s - ./Bitcoin-Qt.app, and open ./Bitcoin-Qt.app) and the app ran with no issues. Having said that, I don't see an issue in having to run those commands in order to make notifications work when compiling the code