Display progress for each file in a file upload request.
Work on a copy of the headers in _initXHRData

If someone passes a "headers" options to fileupload(), this object get overwritten across multiple files upload because the same object is being re-used.
Merge pull request #2550 from rslinckx/patch-1

Work on a copy of the headers in _initXHRData
Refactored last merge and updated version number.
Updated package version.
Check if a context node is given.
Updated package version.
Minor event handler refactoring.
Allow to define the dropEffect and preventDefault handling in the dragover handler. Closes #2557