Bug 1105976 - [Stingray][Home] Add Folder structure and its data access methods

https://bugzilla.mozilla.org/show_bug.cgi?id=1105976
To use `for...in` on array is a bad practice. It is much slower, and it doesn't guaranteed to preserve element ordering.
Please see  https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...in