Terrain tile features.
All possible terrain tile features, some of them parameterized or dependent on outside coefficients, e.g., on the tile secrecy value.
triggered by ascending
triggered by descending
triggered by opening
triggered by closing
triggered when the tile's secrecy becomes zero
causes the effect when triggered
transitions to any tile of the group when triggered
actors can walk through
actors can see through
is lit with an ambient shine
sustains the effect continuously, TODO
items and stairs can be generated there
is a (not hidden) door, stair, etc.
used for distinct paths throughout the level
discovering the secret will require this many turns