Add sign

[BenCheung0422]

Resolves #351 The original pull request doing this is #366 and has been closed because of the incompatibility of the tile handling system. Currently, there is a way do that by adding a separated save file for signs, but it still needs to be combined into a standardized system of tile data save system in the future. Signs could make more contents within structures when complementing the storyline. As mentioned in #542, tile entities could be made with signs.