This makes it easier to spot syntax errors when editing the
class reference. The schema is referenced locally so validation
can still work offline.
Each class XML's schema conformance is also checked on GitHub Actions.
* New plugin system to control the whole import workflow
* Can add options and run code at every import step (general, per node, mesh, animation, material etc.)
This constitutes a first version of these plugins. The ability to interact with the import preview dialog will likely be added later on.