Make grid a resource

[BenjaTK]

I think the grid could be changed to a GaeaGrid resource, instead of just a dictionary. This resource would still have a dictionary, but it would allow for easier adding of features and saving.

The pros:

• Could add get_value and set_value methods which would allow for doing stuff like Add layers #25, RandomTileInfo #48 or TileInfoPattern #51.
• As a resource, it can be easily saved and loaded.
• Grid logic would be separated from generation, which is kinda what we've been doing with stuff like separating generation from rendering, and all that.
• Could have helper functions such as getting the neighbors of a tile (which already exist in the Generator class, but is a bit of a mess).

The cons:

• Lot of refactoring
• Resources can sometimes be a bit wonky
• Can't think of anything more tbh