Level selector was already working after work by William but some small bugs remained to be fixed + it wasn't possible to return to level selector once we were in the level.
- it's now possible to go back to level selector from Level
- LevelManagerScript is no longer a singleton (this way, it resets with the Game scene unloading)
- Added a property drawer for using scene files in inspectors
problème : il faut que les resource tiles produisent des resources seulement quand elles ont une unité au dessus d'elles pour les harvester.
solution : créer une classe harvester qui est vérifiée par la resource tile avant d'updater. Si aucun harvester n'est au dessus, aucune update n'est faite.
logic was supposed to allow placing object on farms, which it didn't
now it does.
+ changed ObjectPlaceholder's name so it follows the TilePlaceholder's convention
