Remove dimensions support

[JimViebke]

I'm passing on the idea of a 3D gameworld in a text-based game. It presents too many challenges for physics, AI, map rendering, and prose.

• [x] Close any issues related to dimensions.
• [x] Remove C::GROUND_INDEX and C::WORLD_Z_DIMENSION.
• [x] Remove all references to a z-axis in the code.
• [x] Rename Z-stacks to something like room_XML.

[JimViebke]

A lot of this should be done. Check, commit, test, and merge.

[JimViebke]

Removed all known references as of e792a90.