add multselect dnd

this pr adds dnd moving of more than one file if selected:

* it adds the createDragShadow helper that collects the selected files
* the dragshadow is a css styled table containing a row for each file with filename and size
* the code also handles dnd for singlefiles and restores the correct selected class / checkbox if dragging is stopped

I also moved from $.ajax to $.get and handle the result myself instead of using boolOperationFinished. mainly because I then don't have to construct the url parameters with encodeURIComponent. I know @DeepDiver1975 uses that method to update the max file upload size in another PR. I'll add that depending on what gets merged first ;)

@jancborchardt @DeepDiver1975 @MTGap @icewind1991 @tanghus @schiesbn If you want multi select dnd give me your :+1: after testing