Added multi app (frontend/backend) support. (#309)
* - Added multi app (frontend/backend) support.
* Bugfix
* Bugfix: Expanded cache key by Yii::$app->id to differenciate between apps (frontend/backend).
I did not notice, that $module->getUniqueId() returns an empty string when $module is the app itself. Therefore the cache could not differenciate between frontend and backend. Thus resulting in the same routes for all indicated apps.
* Filter out empty parent items. Sometimes a menu item is defined by url = # and is used as a container for sub-items. If all the subitems are forbidden then this patch removes the parent entry as well.
* Bugfix: Debug bar disappeared. Thanks to yongtiger (see https://github.com/mdmsoft/yii2-admin/pull/309#issuecomment-280197343)
