Prevent adding a file twice, setting "prevent_duplicates"

As described here http://www.plupload.com/punbb/viewtopic.php?id=571 and here http://www.plupload.com/punbb/viewtopic.php?id=446 we need a way to prevent adding the same file twice.

That's why I introduced a new setting called "prevent_duplicates" which can either be true or false.

It works by using an object to store all filenames as keys to create a lookup-table of already added files. Now when we add a new file we can easily check if the filename is already in our queue. This is simpler and more performant than using nested loops.