💡 simplify entity instantiation

[bitbrain]

Implements #82

This pull request implements proposal #82 (comment) by simplifying the instantiation of entities (e.g. to save them into a custom save game outside of Pandora)

Highlights

• removed PandoraEntityInstance
• custom scripts can now be instantiated
• simplified loading/saving of PandoraPropertyInstance by only persisting it when it actually overrides a value

Usage Example

var entity = Pandora.get_entity(Tools.PICKAXE) as Tool
var pickaxe_instance = entity.instantiate()
pickaxe_instance.set_float("durability", 0.9)
var json_data = Pandora.serialize(pickaxe_entity)
# store json data somewhere!
# pickaxe instance is of type `Tool`, not just `PandoraEntity`
pickaxe_instance = Pandora.deserialize(json_data)

[mrfelipemartins]

Yeah everything looks fine now! Great job! I still have some issues during load but now is all on my side. I can confirm everything on Pandora's side is working properly now.