qt: Use SynchronizationState enum for signals to GUI

Summary:

This PR is a followup of #18121 and:

    addresses confusion about GUI notification throttling conditions
    removes isInitialBlockDownload() call from the GUI back to the node
(on macOS).

Backport of core PR18152.

Depends on D8334.

Test Plan:

ninja all check-all

Also run the tests on OSX.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8335