New feature: manual adjustment of the quadrilateral

[pynicolas]

Related to issues #59 and #72. Re-uses code from PR #164.

Big kudos to @PhilLab who wrote the complete UI for the new screen! Thank you!

[nekohayo]

I gave it a try as it shipped in version 1.21.0 now, and I'd like to thank you and @PhilLab for not only implementing this much-awaited feature, but also in a way that seems to be flawless! The draggable corners and the precision magnifier seems to suffer none of the bugs I've seen in other applications (including the flagship scanning application from that big design apps company…):

• the corners allow precise motion (no jerking around when starting motion), and;
• they don't yank away and misplace themselves when you release the finger from the adjustment motion.

Fantastic!

[pynicolas]

@nekohayo Thanks a lot for the detailed and positive feedback!

It's great to hear that the interaction feels smooth and precise in practice. Huge kudos again to @PhilLab for the excellent UI work on this feature.