1.1.16 branch - useful custom prefix Id generator

According to 
https://github.com/yiisoft/yii/issues/3618
and 
http://www.yiiframework.com/forum/index.php/topic/59046-cache-and-widget-ids/page__p__265719__fromsearch__1#entry265719

I thing it is usefull to include public $prefixId = 'yw'; in CWidget.php