Support for metadata on shape frames

[Goutte]

This only needs a public endpoint to access the current shape frame, if any.

Rationale

I'm also using this plugin to add extra information on each animation frame, and it's been delightful for things like is_before_slash on startup frames.

Using this in AnimatedShape2D, and a get_meta on the result when not null:

func get_current_shape_frame() -> ShapeFrame2D:
    var animation_name := self.animated_sprite.get_animation()
    var frame := self.animated_sprite.get_frame()
    return self.shape_frames.get_shape_frame(animation_name, frame)

The metadata records are added in the Editor, as metadata of a single ShapeFrame2D. Shallow and deep copies seem to work as expected.

You don't have to use a special metabox to use Metadata, you can use the hitbox directly for example, but I've found it to be less messy.

[Goutte]

See d5313849cae6066d5dc15382777ec39687d33e26 .