Thumbnail generation in ListView and download from server

This is the most important point. 

We need to use a single network library in all the app, and this is not. Geting the credentials from and OwnCloudClient and use them in a different client is not a good solution.

There are other properties of the OwnCloudClient state that should be used. Cookies are already supported, although disabled for the moment.

I would recommend to remove (or comment) all the block about download for the moment to get the thumbnails for already downloaded files ASAP. Making this in the right way won't be immediate, since probably changes are needed in the ownCloud client library.