Add "minimum height" option

[jvasileff]

It would be nice to have a minimum height option. I've had success printing credit card size QR codes (width 54mm, error correction low), but the output is too short. Vertical padding would be added as necessary to meet the minimum height. Ideally, the location of the padding would be selectable with checkboxes - above qr, below qr, and below text.

Alternately (perhaps easier?), would be to report the final height and width dimensions in the UI after clicking "generate UI model", and add three vertical padding options. The user could do the math and experiment with settings to get the best result.

(BTW, really great and well done project, thanks for sharing!)