Fix layer box not clearing after last image unloaded
Add some to-do notes in the PDI save function

I don't want to waste too much time screwing with PDI files just yet.
There will be plenty of time for that once PD is back in a working
state.

That said, I've jotted down some quick notes on items that still need
addressing.  But for now, I think things are stable enough for me to
return to working on Layers integration.