Add cropping functionality

[skvark]

Allow user to crop the image before processing.

[skvark]

Needs some small polishing:

• too small crop areas cause error in preprocessing
• UI might need some changes
• if user did not move the corner points, code still crops -> needs fuzzy compare

[skvark]

Some UI polishing required, other stuff fixed in 8b0808d45c and 427d68378cbce238ba335bcfaab7982b6974aadd.