cBlockArea offset

I also added these functions.
```cpp
// Set
void cBlockArea::SetOffset(Vector3i a_Offset)
void cBlockArea::SetOffset(int a_OffsetX, int a_OffsetY, int a_OffsetZ)
// Get
Vector3i cBlockArea::GetOffset(void)
// wich returns the offset.
```

The offset gets loaded and saved from an schematic file.