don't crash when destroying the shortcuts

as shortcutDestroyed(..) modifies the shortcuts list.
disconnect from shortcutDestroyed() first, or operate on a detach()-ed list

this was uncovered by QList::removaAll() optimization patch.

(cherry picked from commit e95de30977291a251660f72baa84b5ff244711fb)

Change-Id: Ie0b371c54458ce85f4d36430de8080a5e78d8b7f
Reviewed-on: http://codereview.qt-project.org/4585
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>

tests/auto/qshortcut/tst_qshortcut.cpp

@@ -1124,7 +1124,8 @@ void tst_QShortcut::context()
 // ------------------------------------------------------------------
 void tst_QShortcut::clearAllShortcuts()
 {
-    qDeleteAll(shortcuts);
+    QList<QShortcut *> shortcutsCpy = shortcuts;
+    qDeleteAll(shortcutsCpy);
     shortcuts.clear();
 }