Gravity

[Insineer]

Add a bool property "gravity" to a tile (In the future, tiles will be combined into Locations and gravity will be the property of location).

• If there is "gravity" and there is no floor, then objects should fall to Z-level below, if it exists.
• If there is no "gravity" then mobs can move by Z-levels upward/downward with shift+W/S buttons if there is no floor/ceiling accordingly.