Implement Layer -> Size -> Reset to actual size
Implement menu-based layer resizing

More complicated than it would first appear, because all resize
calculations have to be handled separately, but the same dialog is used
as Resize image.

I think all nuances have been handled correctly, but additional testing
is welcome.  One note: if non-destructive layer resizing has taken
place, that ratio is maintained after a menu-based (destructive) resize
operation.  I believe this is the best course of action, as the ratio
between the old and new sizes will still be preserved as the user would
expect.