Offer the possibility to validate copy-operation by comparing checksums

Apologies, I am not sure how to tag this as a feature request.

These are some small improvements I think would greatly enhance Thunar's file transfer capabilities without bloating it.

1. With the 4.16.x releases, I humbly suggest that the word "Queued" would be an improvement over "Frozen" for transfers that are held back until current processes are completed. fixed
2. If a file transfer is cancelled, Thunar should automatically offer to delete incomplete transfers rather than leaving part-files. fixed
3. I was playing around with TeraCopy in Windoze and I really liked how you could tick a check box in the copy or move dialogs and have the process verify the file using md5sum (or another) after the move or copy has completed.