Add a function to mark unused chunks instead of just delete them.

[Caverns4]

Certain zones in all the games have hard coded sections that depend on chunk orders, such as oil/water slides, and the Hill Top Zone cliff patches. In those situations, being able to click a button or tick a checkbox to mark unused chunks instead of just delete them would save some time. Marking them with a red or pink background would make sense.

[Pacca64]

Would be helpful for blocks and tiles too, animated zone art in Sonic 2 often needs blocks to be in the right places, and objects that use level art like the GHZ cliffs heavily rely on what sonlvl considers 'unused' art. Having a clear way to dance around these situations would save some time.