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.