Fix QOleDropTarget object leak

1) call Release() to nativeExtra->dropTraget in
QWidgetPrivate::unregisterOleDnd() to prevent leak

2) call RevokeDragDrop() in QWidgetPrivate::setParent_sys() when the old
window handle is to be destroyed and the current widget has already
registered an ole drop target object to prevent leak

Change-Id: I767f4e0ea3e6cb5f14d72ba88e1f8f1bc8c0dc3c
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Doc: update description of qmake SUBDIRS variable

You can specify a relative path to a .pro file in any directory.

Fixed a capitalization issue and a grammar issue and changed the
visible text in a link target.

Task-number: QTBUG-14139
Change-Id: Ib476bd400c46645709950c582492edcaaba9d7ed
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
(cherry picked from qtbase/31e665c7cd9b2edb25fa695bed9c528c95ad0c64)