Allow overriding any default GOAT scene

[Miskatonic Studio]

Since GOAT became a plugin and moved to Godot v4, a new mechanism was added. It allows users to define custom settings screen (displayed in-game and in the menu), while a default screen is also provided by the framework.

This mechanism should be extended to other GOAT scenes (e.g. 3D inventory, context inventory) to make the framework more flexible.