fixed file separator for windows, diamond notation

Fixed file separation symbol to run cross-platform
Fixed diamond notation and generics
Resolved unchecked warnings - all except one in FileTreeModel which I had to suppress (the cast is safe)
Removed “FileSieve” label
Added minimum width for main frame

Please let me know if it's Ok to merge. Will work on the rest on Wednesday - Thursday. Thanks.
This line assumes the user was aware that he/she was only supposed to have used the file chooser to select an existing folder within which to place the diff report (whose name is hardcoded here).  Per Hangouts discussion, I have posted sample code in out Blackboard discussion area for a JFileChooser that, "enforces the selection/specification of a file name with the enforcement of one of a number of specific, predefined 3-character extensions."