Nudge the selected layers around the canvas with the arrow keys

[Keavon]

The arrow keys nudge (move) the selected layer(s) by one canvas pixel. When the Shift modifier key is held down, the nudges go in increments of 10 canvas pixels.

This is probably a good first issue to get acquainted with some concepts in the Rust backend. Please reach out to @TrueDoctor here or on Discord for in-depth instructions to help you get a feel for the code that this touches.

[henryksloan]

I've started working on this.

[Keavon]

This is blocked on the inability to transform layers directly, rather than just folders (issue #243).

Some discussion from Discord:

@henryksloan: Operation::TransformLayer seems to act on Folder layers; is that the wrong way to move an individual layer?

@TrueDoctor: That is a shortcoming of the current design. We already have a fix, it just needs to get implemented.

[Keavon]

@henryksloan you should be unblocked now courtesy of the swift work of @0HyperCube.