QtWidgets: Fix build & warnings with QT_NO_GRAPHICSVIEW.

kernel\qapplication.cpp(3157,18) :  warning: unused variable 'isGraphicsWidget' [-Wunused-variable]
            bool isGraphicsWidget = false;
                 ^
kernel\qwidget.cpp(6446,18) :  warning: unused variable 'window' [-Wunused-variable]
    if (QWidget *window = w->window()) {
                                          ^
kernel\qwidget.cpp(7949,50) :  error: no member named 'proxyWidget' in 'QWExtra'
        if ((q->isWindow() && (!extra || !extra->proxyWidget))
                                          ~~~~~  ^
kernel\qwidget.cpp(8084,50) :  error: no member named 'proxyWidget' in 'QWExtra'
        if ((q->isWindow() && (!extra || !extra->proxyWidget))

Change-Id: I8474ab0ab4617c6588707ce0c2f7a97e4d0e54da
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>