added sorting links to debug toolbar

as requested in
https://github.com/yiisoft/yii2/pull/1781#issuecomment-31587963
https://github.com/yiisoft/yii2/issues/1478#issuecomment-31576474
fixes #1478
Do not unset class properties!

this will result in objects that do not have the property anymore :-)
try to fix basic app travis test.