Implement drag & drop

* remove a lot of code (but not all) associated with the old uploading widgets
* implement drag&drop via plupload - http://www.plupload.com/
* implement chunking to prevent larger uploads from crashing the browser
* add a json format to the manager for better communication about uploads

This is a first step, but there is still a lot of work that needs to be done to improve/test this.