(svn r3046) Replace some numbers with sprite names and fix a typo in the sprite list

(svn r3045) -NewGRF, Codechange: Don't attempt to map and empty sprite group to a vehicle.

(svn r3044) -NewGRF, Codechange: Improve handling of non-existent sprite sets.

(svn r3040) Don't deduce the cursor sprites when building rails from magic numbers, but fetch them from the central rail info array.

(svn r3039) Revert accidently commited changes from last commit

(svn r3038) Reorder the loading of standard graphics files to reflect a bit where in the sprite array the sprites end up and assert, that the indices are equal to the corresponding sprite base enums, to guard against typos.

(svn r3037) Don't deduce the sprites for tunnels and level crossings from magic numbers, but fetch them from the central rail info array. This is a preparation step for electrified rails

(svn r3036) Use IsCompatibleRail() to check the rail type of level crossings (not a fix, because the old == method works so far)

(svn r3035) Augment the engine table with symbolic names for rail types and climates

(svn r3034) -NewGRF: Improve error checking of setting price bases.