QApp: Method to check for native style usage.

Adds QApplicationPrivate::usesNativeStyle to check whether the
QApplication is using the native platform style.

This can be needed internally to decided whether to let the platform
plugins do stuff or do it interally, style dependent. E.g. letting the
platform plugin popup a QMenu vs. letting the style draw one.

Change-Id: Ibb5e11a4d9d1d2824685ff146786a9354ceef43c
Reviewed-by: BogDan Vatra <bogdan@kde.org>
qfreelist: fix data race on v[at].next

Detected by clang's -fsanitize=thread in tst_qcoreapplication.

Task-number: QTBUG-39024
Change-Id: I60b7cece0384f89dc62ac5128faf39a4084e72e2
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Doc: Fix undefined variable in QMetaMethod::invoke() documentation

Task-number: QTBUG-38871
Change-Id: I5b7aef58c652d13110aa73b7856927b5ba6fea50
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>