Fix parent class for FlagColumn

Maybe I'm missing something, but I checked this out, if we extend our FlagColumn class from CDataColumn instead of CGridColumn we don't have to copy renderHeaderCellContent method and renderFilterCellContent (to make filter working)
Also, it seems to be a bug in a book https://www.dropbox.com/s/csu2zugwi15e1un/yii-bug.png