Add buttton to stop imaging manually

[bitsgalore]

In case of a seriously damaged disk, imaging with ddrescue may take an excessive amount of time. So perhaps add a button that allows an operator to stop ddrescue manually. Adapt from diskimgr's "interrupt" button.

Example: HD floppy with bad sectors took over 30 mins to image.

[bitsgalore]

Not sure if this is a good idea. If excessive read times are a problem, a better solution might be to reduce the retries config setting (default: 4).

It might be helpful to display some ddrescue progress info though, because right now Ipmlab doesn't provide the user any feedback on what ddrescue is doing.