This is a partial revert of D513. It is highly unlikely in 2024 that anyone will directly upgrade from v0.14.6 to 0.28.12 without any intermediate version. And I don't see a good reason to use Core's settings for a user who first installs Bitcoin ABC.
The affected settings are mostly unimportant GUI state (window geometry, fonts...), and a few parameters that we probably don't want to copy anyway (data dir location...).
```
("DisplayBitcoinUnit", "MainWindowGeometry", "PeersTabBanlistHeaderState", "PeersTabPeerHeaderState", "RPCConsoleWindowGeometry", "RPCConsoleWindowPeersTabSplitterSizes", "RecentRequestsViewHeaderState", "SubFeeFromAmount", "TransactionViewHeaderState", "UseEmbeddedMonospacedFont", "enable_psbt_controls", "fCoinControlFeatures", "fFeeSectionMinimized", "fHideTrayIcon", "fMinimizeOnClose", "fMinimizeToTray", "fReset", "fRestartRequired", "mask_values", "nConfTarget", "nFeeRadio", "nSettingsVersion", "nSmartFeeSliderPosition", "nTransactionFee", "strDataDir", "strThirdPartyTxUrls")
```
In this diff I preserved the `setFallbacksEnabled(false)` behavior on MacOS that was introduced in D513. The rationale for doing this is not clear from the summary of that diff, so better to keep it just in case it really serves a purpose. Nobody complained about it since it was introduced.
From the QSettings documentation:
> Fallback Mechanism
>
> Let's assume that you have created a QSettings object with the organization name MySoft and the application name Star Runner. When you look up a value, up to four locations are searched in that order:
>
> a user-specific location for the Star Runner application
> a user-specific location for all applications by MySoft
> a system-wide location for the Star Runner application
> a system-wide location for all applications by MySoft
>
> If a key cannot be found in the first location, the search goes on in the second location, and so on. This enables you to store system-wide or organization-wide settings and to override them on a per-user or per-application basis. To turn off this mechanism, call setFallbacksEnabled(false).