Send chunk arguments, when chunking is enabled. Related #816.

No matter if file is bigger than chunk or smaller.
UI: Fix regression - start_button should get enabled if more files are added to the queue.

For "multiple_queues=false" case browse_button will get disabled, so no
files will be added. At least not through the UI...
UI: Add form fileds only once on UploadComplete. Address #830.

Historically filelist was rebuilt on every change in queue, be it new
file or file status change, so the form fields were automatically
always correct. Now that we do not redraw the filelist now and then, we
have to do it manually and properly.