Make Separate Channels work again

* This removes the code for saving the separations to files. This
has been disabled since 2006
* Fixes adding the layers (this was broken because the progress
reporter was set to 100 before adding the layers, which means that
iterrupted() was true, so the layers were never added)
* Fixes unlocking the image (which happened inside the block that
added nodes, which wasn't executed because of see above)
* And makes it possible to only activate the relevant channel when
separating to color

BUG:336694,412624