Use pointers for QAccessibleEvent.

Use pointers for QAccessibleEvent.

At least on mac g++ badly wants to copy the event and
cannot use the copy ctor. The sensible solution is thus
to use pointers. This is in line with QCoreApplication::sendEvent.

Change-Id: Icb58852be351ab04ffa17069989d7a07d4b377da
Reviewed-by: Jan-Arve Sæther <jan-arve.saether@nokia.com>