This enforces decoupling and in the case of QMacStyle, isolates
QtWidgets and therefore end user applications, from Carbon/HITheme.
Windows and Fusion are platform independent, so they remain built-in
(but mostly because the Windows style is tightly coupled to other styles
like QStylesheetStyle).
Task-number: QTBUG-59428
Change-Id: Id6519fe0c5269c1bce5b5921f9db06257032a1c9
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@qt.io>
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