Generating setters for indexed properties, like SpatialMaterial::albedo_texture

[ghost]

As Rust has no language-level notion of properties, the properties list is currently simply ignored, with the assumption that each property would have a corresponding set_ method in the API. However, the Godot API also contains "indexed" properties that wrap around other methods:

{
	"name": "albedo_texture",
	"type": "Texture",
	"getter": "get_texture",
	"setter": "set_texture",
	"index": 0
},

While such properties are still possible to access, using the indexed methods directly, they do not currently have convenience getter/setters, which can be confusing to users: see #688. As a result, we might want to find those properties and generate additional methods for them.