Add wall versions of beta terrain edges

[fidwell]

Resolves #340. Created new wall objects that match the texture of the two beta terrain edges.

I wasn't sure what to name these. The terrain textures are called Brick (Brown) and Rock (Brown), so I went with Brick wall (Brown) and Rock wall (Brown) to match. Open to other suggestions.

For the object ids, the terrain textures are rct1beta.terrain_edge.brick and rct1beta.terrain_edge.rock, but the walls aren't from the beta, but maybe the names should match anyway? They're easy to change regardless.

[ZeeMaji]

I'm a bit unsure if it would make sense to include the rock walls as there's no wall version of the final/RCT2 rock edge.

For the names i would just shorten them to "Brick Wall" and "Rock Wall" or even just "Wall" as the wall names in vanilla are pretty generic.

For the object ids should remain "official" as they were not a part of the rct1 beta, they're just official additions based on the beta assets.

[fidwell]

I didn't even consider that some of the terrain edges don't have corresponding walls. I probably wouldn't have made the rock wall if I'd thought about it.

If anyone feels strongly about not having them, that's fine by me.

[tbarford]

Maybe add that wall texture as a scenery wall, too?