Make it possible to circulate focus between QGraphicsProxyWidget's in a QGraphicsView

Test case main.cpp to reproduce issue
//-------------------------------------------------------------------------
#include <QtGui>

int main( int argc, char * argv[] )
{
QApplication app( argc, argv );

QGraphicsScene scene;

QLineEdit * box1 = new QLineEdit;
QLineEdit * box2 = new QLineEdit;

QGraphicsProxyWidget * proxy1 = scene.addWidget( box1 );
QGraphicsProxyWidget * proxy2 = scene.addWidget( box2 );

QGraphicsGridLayout * layout = new QGraphicsGridLayout;
layout->addItem( proxy1, 0, 0 );
layout->addItem( proxy2, 1, 0 );
layout->setColumnFixedWidth( 0, 100. );
layout->setColumnFixedWidth( 1, 100. );

QGraphicsWidget * form = new QGraphicsWidget;
form->setLayout( layout );
scene.addItem( form );

QGraphicsView view( & scene );
view.show();

return app.exec();
}
//-------------------------------------------------------------------------

The focus should not be transferred to another proxy when pressing Tab. It is the same behavior on the desktop. Pressing tab does not transfer the focous to another window, so it is more a "make it possible to switch between proxy widgets using Ctrl+Tab" suggestion, similar to task 169874.