Remove QGraphicsProxyWidget crash in QWidget::hasFocus().

A QGraphicsProxyWidget embeds a focusable widget (e.g., QComboBox). When
deleting QGraphicsProxyWidget, the QWidget will be deleted. The QWidget
clears focus, and QWidget::hasFocus() is nice enough to check if its
embedder QGraphicsProxyWidget has focus - because if it does, it wants
to clear focus from that item too. QGraphicsItem's destructor already
calls clearFocus() however, so this call is unnecessary; we can simply
stop clearing the QWidget's focus in its destructor if the widget is
embedded.

QWidget::hasFocus checks QGraphicsItem::hasFocus (on the proxy widget
that is being deleted), which checks its d_ptr, which is gone. It's
generally unfavorable for an object deleting a child to have the child
go back and poke at the parent object, which is in many ways what's
happening here.

Task-number: QTBUG-29684
Change-Id: I1e52bf28f47b2824752de28dff2d0de13733ee48
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

src/widgets/kernel/qwidget.cpp

@@ -1391,6 +1391,14 @@ QWidget::~QWidget()
 
 
     QT_TRY {
+#ifndef QT_NO_GRAPHICSVIEW
+        const QWidget* w = this;
+        while (w->d_func()->extra && w->d_func()->extra->focus_proxy)
+            w = w->d_func()->extra->focus_proxy;
+        QWidget *window = w->window();
+        QWExtra *e = window ? window->d_func()->extra : 0;
+        if (!e || !e->proxyWidget)
+#endif
         clearFocus();
     } QT_CATCH(...) {
         // swallow this problem because we are in a destructor