All calendar backend accounting was moved into QCalendarRegistry class
(renamed from Registry). Calendar backends are no longer registered
inside constructors, because in multithreaded environment
this may lead to incompletely initialized instances becoming visible
via QCalendar API in multithreaded environment. All system backends are
registered by QCalendarRegistry itself when they are needed. New method
QCalendarBackend::registerCustomBackend() is provided to register
any 3rd-party calendar backends.
Registration by names was also simplified. The list of names is now
passed to QCalendarBackend::registerCustomBackend(). The checks are
provided to ensure that all system backends have non-conflicting names.
Name conflicts for custom backends are resolved in favor of earlier
registered backends, as it is already the case in the existing code.
The documentation was updated to reflect that.
Method QCalendarBackend::names() was added to query the list of names
associated with a backend after it is registered.
Calendar backend deregistration was completely removed because it is
not possible to perform it safely without reference counting.
Fixes: QTBUG-93004
Pick-to: 6.2
Change-Id: I0ab1eccc02fdd1e1c66b5e5dd076c93de32d5a49
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
d0ae1ef33a