A simple way to cancel out of a currently-in-progress-huge copy/move operation

[older-pack]

Checklist

• [X] I made sure that there are no existing issues - open or closed - to which I could contribute my information.
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• [X] I have read the FAQs inside the app (Menu -> About -> FAQs) and my problem isn't listed.
• [X] I have taken the time to fill in all the required details. I understand that the bug report will be dismissed otherwise.
• [X] This issue contains only one feature request.
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Feature description

Some kind of cancel method to stop a copy/move operation.

Why do you want this feature?

Having a way to cancel huge file copy move operations might be useful. Having made the mistake mentioned in #137 the only way I could think of stopping moving 2000+ images was to shut down the phone.

This is related to #137 and perhaps if/once #137 gets implemented this feature might be a bit redundant? or maybe it would still be useful for people to change their minds about a move operation?

Additional information

Perhaps having some kind of filter where the option only appeared if the number of files being moved was greater than a certain threshold: 50, 100 or 200 images? and/or an entire folder?