List the assets to load + tilesets in json

[milkyroute]

I thought it would be interesting to have a list of assets to load in json instead of having them directly written in the code. That would allow more additions only with the json files (For example, we could directly add a new NPC with textures and everything simply with json).

[milkyroute]

Extended the issue to tilesets too. Adding a multiple tileset system will be required.