About the progress dialog, I'd like to propose a change.
In the Qt version, I showed the files being processed in a list box rather than showed them in a single line.

Instead of current design:
Copy: file1, file2, file3, file4, ....

I list them in a listbox with a scrollbar.
|  file1   |
|  file2   |
|  file3   |
|  file4   |
So it became clearer what the files are and their full names can be shown properly. I'd suggest do the same in the gtk+ version.