Improve Qt::WA_UnderMouse accuracy

Improve Qt::WA_UnderMouse accuracy

Qt::WA_UnderMouse is set/cleared when widgets get enter/leave events.
When there is a popup active, Qt::WA_UnderMouse should always report
false, but this was not happening, because existing state was not
cleared when popup was opened.

Dispatch a leave event for last mouse receiver when a popup is
opened to update the Qt::WA_UnderMouse state. This is roughly
equivalent to what happens on Qt4.

Task-number: QTBUG-27478
Change-Id: I7739e75727213e748ab2f42f1027d32325d89fb0
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>