Adding LevelItem culling, Duplicate To expansion, etc.

- Renamed Camera.cs file to EditorCamera.cs to more accurately reflect
it's contents
- Shortened the default draw distance to a useful, faster value
- fixed some drop-down controls, per MainMemory's request
- Added experiment camera frustum culling. Code is still WIP.
Merge branch 'master' of https://github.com/sonicretro/sa_tools