[Suggestion] Add option to disallow enabling tile stacking in the layer settings, per layer

[FaultyFunctions]

I think this would prevent accidental tile stacking and also wouldn't be too hard to implement, since it would just be a check for the setting to allow or disallow it.

This would be super useful to again prevent accidental stacking and ensure workflows where a layer counts as only "one depth" layer, if that's the constraint users want to achieve.

The only concern I can see arising is what happens if you already have stacked tiles on the layer and you disable the ability to enable tile stacking on the layer? I can see two options here:

1. You just disable the tile stacking option on the layer, and don't allow to re-enable it unless the actual layer setting allows it. Previously stacked tiles would stay and placing new tiles over stacked tiles would work the exact same as how it already works if tile stacking is disabled. This would be the simplest and the option I'd probably lean towards because it is non-destructive.
2. Warn the user that toggling this option will delete all "stacked" tiles above the base tile. Obviously this would be destructive, and would require the code to keep some sense of the "stack depth" of each tile. This would be more complex because I'm not sure if LDtk has a concept of "stack depth" already coded in. Would still recommend leaning towards the first option.