Copy event

[gurgalex]

Hiya!

Neat project. Great for looking through full screen images.

I've added the Copy functionality (mostly copied the move_image code).

I've added a reminder to decide what to do when the image already exists (may not want to override the image). Prompt if no option specified or use specified option may work.

Fixes #21

[gurgalex]

Hmm, should probably move getting the current filepath and making the destination path to a function. Noticed that when looking at how to delete a file next.

[gurgalex]

Update: Fixed Do not merge. There seems to be a bug where copying gets called without a keypress when 0 images are present.