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.
This provides better usability when a GradientTexture or CurveTexture
is added to a Control node.
Visual appearance of most GradientTextures and CurveTextures will
be unaffected.
Neither name is a perfect match but `Curve3Texture` looked too similar to
`CurveTexture` and `Curve3D`, which made things confusing when picking a
texture type or browsing the API reference.
