Overwrite requester

[MM-Mikhail]

Missing ability to overwrite files if they already exist, or skip.

Also missing ability to abort operations.

[chris-y]

Aborting requires the use of XAD hooks.

• [x] Support XAD hooks
• [x] Add Cancel button
• [x] Support overwriting existing files

[chris-y]

Looks like the progress hook is needed for both cancel and overwrite.

[chris-y]

Need to be more careful with my commit message phrasing :)