- Add MIME definitions for resources, scenes and scripts
- Remove the "weight" property, which defaults to 50, a much saner value than the previous 100, which was a bit excessive.
- Changes their icon names in order to follow the XDG icon naming conventions.
Mime type definitions are usually named like the mime type they define,
i.e. x-godot-project in our case.
It was introduced with reverse DNS naming as that's what FlatHub favors,
so it will need to be renamed there.
