Editor: Undo a batch of setpixel actions

[neoneye]

Problem

Undoing set pixel actions happens one pixel at a time, so it takes a long time to undo the latest drawing operation.

Solution

The entire history of the set pixels actions must be preserved, since it's needed for replaying the solution.

Idea: assign a unique id to the actions that belong to the same batch.

[neoneye]

I'm keeping the undo as it is. Having solved several ARC tasks with the editor, I don't think batch undo will add much value.