Symbian: Fullscreen softkeys responsiveness needs binary compatible reimplementation

This code was removed from src/gui/kernel/qapplication_s60.cpp and src/gui/kernel/qwidget_s60.cpp:

		// Responsiviness
		CEikCba *cba = static_cast<CEikCba *>( buttonGroup->ButtonGroup() ); // downcast from MEikButtonGroup
		TUint cbaFlags = cba->ButtonGroupFlags();
		if(qwidget->windowFlags() & Qt::WindowSoftkeysRespondHint)
		    cbaFlags |= EAknCBAFlagRespondWhenInvisible;
		else
		    cbaFlags &= ~EAknCBAFlagRespondWhenInvisible;
		cba->SetButtonGroupFlags(cbaFlags);