Add per-Tile "Allowed Transformations" overrides

As of 1.5, Terrains can now automatically transform (flip horizontally/vertically and rotate) tiles to expand a terrain's capability and/or provide additional visual variation. However, these Allowed Transformations are set for the whole tileset, and that isn't enough granularity for many tilesets.

For example, cliff tiles usually have only one acceptable orientation because of their shading, while grass and sand can often be flipped horizontally, and some simple sand/dirt tiles can be flipped vertically as well. Pipe tiles might be rotatable, while fences are not.

Being able to set Allowed Transformations on a per-tile basis, overriding the tileset-wide setting, would make full use of such tilesets much easier, allowing users to get a lot of mileage out of the flippable tiles without risking nonsense results with the unflippable ones.

No files or tests are named. Locate the existing tileset-wide Allowed Transformations handling, then trace how terrain matching reads it and how tile properties are edited and saved. Done means a tile can override the tileset setting while tiles without an override continue using the tileset-wide value.