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.
Having a property which has the same name as its class leads to confusing
situations (e.g. `BaseButton` has a `shortcut` property of type `Shortcut`
which has a `shortcut` property of type `InputEvent`).
Also renames `is_event` to `matches_event`, and `is_valid` to `has_valid_event`
to better reflect what the methods check.
For the time being we don't support writing a description for those, preferring
having all details in the method's description.
Using self-closing tags saves half the lines, and prevents contributors from
thinking that they should write the argument or return documentation there.
