don't we need some kind of folder browser just like the nbviewer of ipython ?

What is the status of this pr now? Are we still working on it?

I'm not working on it at the moment.
Basically this PR works, but it need to be polished and there're some bugs.
If somebody have better front-end skill can look at it, that would be really helpful.

@Leemoonsoo I am working on some refinements of this pr on this branch: https://github.com/zhongneu/incubator-zeppelin/tree/test-folder

There are still some bugs:

1. need a better "folder opened" icon. Any recommendation on this? it seems the current icon library doesn't' include one...
2. the mouse cursor is not correct over folders on homepage
3. the first item inside a folder in the pull-down list doesn't work well

I have almost no experience in front-end development, so any guidance will be really appreciated.

@Leemoonsoo @corneadoug I've made a coupe of fixes in my branch, and now it works pretty well, but still have one problem: we can click the folder in dropdown, and goes into an empty notebook. I am not sure about how to block clicking on a folder properly. Besides, the css styles for dropdown-menu and dropdown-submenu seems a little bit mess. I am not sure whether it is OK.

I don't want to introduce any bad smells into the frontend code, so I will be really appreciate your review and guidance. Besides, feel free to merge my work into this PR and continue working on it.

@zhongneu How about creating a PR instead?
It will be easier to comment on the code. You can always base your PR on this one.
Once it's done, ping me in it and I will check your code and try to provide some guidance, and also try to provide some solutions to your problems.