add initial support for dropping file through repo tabs

Drag your file into the target repository in the repo tabs
And it is done after a confirmation prompt for the user

Edited: 
Uploading:
![screen shot 2014-11-26 at 11 42 09](https://cloud.githubusercontent.com/assets/2123461/5196021/501ca20c-7561-11e4-9e2e-508b704c1eee.png)
Failed:
![screen shot 2014-11-26 at 11 42 19](https://cloud.githubusercontent.com/assets/2123461/5196027/5cdaafc0-7561-11e4-9d73-c5ca89d9d179.png)

`getRepoItem` can return either `RepoItem` or `RepoCategoryItem`, so `static_cast` is not proper here.