Added BuildZone

- Added a large cube (BuildZoneVolume) with a simple time-starting
script on it.
- Also added another large cube (NoBuildZoneVolume) which disables
placing objects on the wrong side of the river.
- Building Objects timer starts when object is grounded (has hit
TerrainCollider), meaning that they are kinematic after 5 seconds on the
ground
- Misc minor fixes/improvements