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.
Most of the custom logic to handle special keying cases is now inside
the AnimationPlayerEditorPlugin.
The EditorInspector now emits a signal when inspecting a new object.
The second parameter of the signals `EditorInspector.property_keyed` and
`EditorProperty.property_keyed_with_value` can be NIL, causing the event
to fire with less arguments when using `emit_signal` that accepts
Variant arguments, so we use the pointer version instead.
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.
We already removed it from the online docs with #35132.
Currently it can only be "Built-In Types" (Variant types) or "Core"
(everything else), which is of limited use.
We might also want to consider dropping it from `ClassDB` altogether
in Godot 4.0.
Follow-up to #31925, `<member />` tags just before `</members>` would cause
a parsing issue, and we'd never notice that we're no longer parsing members.
Also added space before closing `/>`.
