Undo/Redo: diffs now implemented for standalone layer headers

What does this mean?  Any action that modifies only a layer's header
(e.g. moving the layer around or non-destructively resizing it
on-canvas) will now diff *only the layer header* when writing Undo/Redo
data, making such actions extremely fast and lightweight.

To properly test this behavior, I have also added moving and
non-destructively resizing layers to the central processor, meaning all
such actions now participate in the Undo/Redo stack.