Do not let different slot types be used at the same coordinates and time
Resolves #32
Hello! This does exactly what the commit messages states, multiple slot types may share the same coordinates, but obviously if one (e.g. sitting slot) is occupied, another object may not use the same coordinate to lie down – it is already occupied.
This is important for the upcoming work on the sleeping mechanics, the player will still be able to just passively let the characters lie down on the bed, but also to let them sleep on it – obviously the two slots (lying down vs. job) for that will overlap. One step closer to make that work. 🙂