Terrain Editor improvements

[Son-Guhun]

• [ ] Allow selecting a variation
• [ ] Implement "sticky mode" (mouse movement will only trigger height, won't move brush).
  • [ ] Implement x-only sticky mode (brush will still move in y direction)
  • [ ] Implement y-only sticky mode (brush will still move in x direction)
• [ ] Allow holding a key for sticky mode
• [ ] Implement moving camera with mouse
• [ ] Implement rotating camera with mouse (holding ctrl)