Add Snap-to-edge guides

[RadicalZephyr]

Basically, it's quite difficult to get the edges of boxes that should be vertically aligned to actually be vertically aligned. A snapping effect that when you get within five pixels (or something similar) of another edge that is aligned would be a great benefit.

[howlowck]

Thanks for the suggestion. The feature would add a lot of complexity to the logic, but I'll think about how to implement this.