Implement new material system

[SamVanheer]

Implement a new material system that replaces the materials.txt-based system and Condition Zero's materials system with a single unified system for both footsteps and impact sounds.

The materials file has already been converted to JSON, but the system should be further overhauled to use proper material type names (e.g. concrete instead of C) and moving the properties for the material types to a config file as well (sounds, effects created on impact, etc).