QWindowSystemInterface: fix mem leak and race

QWindowSystemInterface: fix mem leak and race

There was a race where QGuiApplicationPrivate::processMouseEvent accessed
QWindowSystemInterfacePrivate::windowSystemEventQueue without holding
QWindowSystemInterfacePrivate::queueMutex.

There was a memory leak where QWindowSystemInterfacePrivate::windowSystemEventQueue
would not delete events contained in it when it was destroyed.

Fix both of these by properly encapsulating the QList/QMutex pair
in a small class, WindowSystemEventList, that allows only properly protected access
to the internal QList and calls qDeleteAll() in its dtor.

Change-Id: Ifaa9968c9272096df2f7109a7a6cf1c8e5fa736c
Reviewed-by: David Faure <faure@kde.org>
Reviewed-by: Jørgen Lind <jorgen.lind@nokia.com>