P: Add a cancel confirmation in Import window

Yesterday I was hit with a specific problem - while trying to use Import window, I was sieving a batch of 4.5k photos. After like about 80% done I missed the tilde key (toggle import status) and pressed ESC. To my surprise there was no confirmation on cancelling the choice, it simply disappeared. With 2 hours of my time.

I switched to using P/X shortcuts and redid that execrcise, but it's a bad idea to allow exiting this window without any visible confirmation (a simple check: if amount of chosen photos > 0 -> show confirmation before exiting import modal).