So many palettes!

This time, I do it the correct way: I start mapping which (hardware) palette is used where so that I can later properly track copies that goes on all over the place and figure out why I can't properly load/store palettes in that "multipal" update.

Many of the operations you can do on the PaletteWindow actually involve copies from one of the palette into the others. Ideally, they are all synchronised...

•  as soon as you start editing your palette, it differs from those of the "upper screen" which are kept static
• "okay" button copy the edited palette into SPRITE_PALETTE_SUB, allowing a preview on the current sprite page
• if you're fine, "sure" button copies the edited palette on BG_PALETTE_SUB as well. It's now officially your working palette for the grid.
• At anytime, you can undo something by clicking "oops", which copies BG_PALETTE_SUB back as the edited palette. 

There's something the multipal approach changes, however: what is now stored in the .spr file is the offscreen set of (up to) 4 palettes, which is updated every time you flip to another slot. So "okay/sure" only affects how you will draw pixels in the close future, not what is actually stored when saving. The bare minimum I can do is to ensure that onscreen->offscreen update is performed when clicking "okay" as well. 

I *could* also enforce this synchronisation when going out of the PaletteWindow, but that would break the former user interface convention (where going out without having pressed "okay" at all means you won't save your palette edits). To overcome this, and avoid data loss, I introduce an additional offscreen undopal[] where I can automatically store data, and a "lost" button that can recall what was on screen the last time you left the palette edition "window".