QObject: don't #include qproperty.h

The qobject.h header needs QBindingStorage in-size and QBindable
in-name-only. The former was moved to its own header in a previous
commit, which we include now, while the latter can just be
forward-declared. This allows dropping the qproperty.h include from
qobject.h.

[ChangeLog][Potentially Source-Incompatible Changes] The qobject.h
header no longer implicitly includes qproperty.h. If your code depends
on the transitive include, explicitly include <QProperty> where
needed.

Task-number: QTBUG-97601
Pick-to: 6.2
Change-Id: I8d6320f5978e20dfc394d2b1e49f626b99529c37
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/kernel/qobject.h

@@ -54,7 +54,7 @@
 #include <QtCore/qmetatype.h>
 
 #include <QtCore/qobject_impl.h>
-#include <QtCore/qproperty.h>
+#include <QtCore/qbindingstorage.h>
 
 #if __has_include(<chrono>)
 #  include <chrono>
@@ -63,7 +63,7 @@
 QT_BEGIN_NAMESPACE
 
 
-class QBindingStorage;
+template <typename T> class QBindable;
 class QEvent;
 class QTimerEvent;
 class QChildEvent;