Bug fixes

I fixed most issues marked as bug.

* #125 [chrome] Bug when drop folder
* #168 Batch deletion is not working
* #170 Adding serveral times the same set of files gives a progress "NaN"
* #171 Tooltip for adding new files displays previously uploaded files name
* #176 Filename cut when download (Firefox)
* #178   After a batch action, the select-all checkbox is still checked

I also fixed a minor localization issue ("upload a folder" text link was not localized).

I added tests were I already have some and because other issues were related to upload which is something non-trivial to write tests for and will be the subject of a separate PR.