file-operations: Check before copy/move for 4GiB limit on FAT

Nautilus attempts to copy/move >4GiB files to a FAT volume and fail once it reaches the 4GiB threshold. The verify_destination() function already scans all of the source files before a copy / move, so we can add a 4GB check with almost no overhead. Add attributes to the SourceInfo to track the largest file. If that file exceeds 4GiB, warn before initiating the operation. Similar to the existing before of copying to a volume with insufficient space, allow forcing the copy.