Add four corner manipulation to Preview papers

[brettfiedler]

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[jessegreenberg]

An initial commit for this has been added. Here is how it works:

1. Each debug program has a cut button in the bottom left.
   

2. That breaks it into the four corners that can be dragged around like this:
   

3. The corner position for paper-land is at the center of each debug corner.

4. The program can be removed by the close button in the top left corner.

5. Resize and rotation is disabled when cut because those are now possible from moving individual corners (and it would be pretty tough to support cut + resize + rotation).

@BLFiedler over to you! Is this working as you expected and is there anyting you would like to change?

[brettfiedler]

Awesome!

We might want to be able to reverse the process in the event someone accidentally clicks the scissors. Can keep it on the BL corner. Does that seem relatively simple to do? The other option of course is to close the program and bring it back in, so it's not gamebreaking.

Question: Does the actual position of the corner marker align with the very edge of the preview corner?
i.e.,

[jessegreenberg]

OK, a button to recombine programs has been added. When you cut/recombine now it will reset corner positions relative to the top left corner and remove any rotation. That seemed like a reasonable behavior but let me know if it should change.

Does the actual position of the corner marker align with the very edge of the preview corner?

The actual position is at the center of each preview. That was just easiest because each debug corner could be positioned the same way. But it wouldn't be too hard to position each relative to its outer corner like that if that is best.

[brettfiedler]

I think we will want to align the position with the outer corner to better match the paper papers. We'll try to stick to decisions like that to keep as much parity between them as possible.