More small track design refactors

[Gymnasiast]

This brings some more refactors to the in-memory track design structures. The scope is limited because the code is frankly a lot of spaghetti that needs careful untagliatelleuntangling.

• Replacement of snake_case with lowerCamelCase
• Simplification of maze entries (possible now that maze entrances/exit are no longer stored in there)
• Addition of a "tertiary colour" field for scenery (previously only supported by walls and hidden in the "flags" byte)

Note to tester(s): make sure to test maze designs and designs containing a wall with three colours.

[spacek531]

I will update this as I find more. Wall with 3 colors does not work

[Gymnasiast]

Third colour on walls should now be fixed.

[spacek531]

Looks good and works

[mrmbernardi]

Provided my other comment is addressed, everything else looks good.