PR REDO: Below The Surface - Reimplement Tom's guide for Tiled editor

[MHLoppy]

This is a manual re-implementation of #191 to enable merge conflict resolution - please see there for PR info.

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Additional notes:

• The use of Tiled editor has become largely redundant due to the discovery that the BtS team actually did publish their level editor.
• This PR (as in the original) includes a handful of undocumented level changes which I've included but which are not explained in the original PR.