Preparations for graphical editor support

[mbrea-c]

The development of the visual editor needs a number of changes in the library, which has resulted in breaking changes.
Key changes:

• Serialized format of animation graphs changed, to simplify the serialization code (for saving edited graphs back to disks). Existing graphs will need to be adapted by hand.
• Extended AnimationGraph API with node positions data (in Extra), validation, node renaming and node removal.
• Updated node icons to work with egui default font.
• Fixed reflection support for a variety of types.

This PR is already massive so further changes will be left for further PR:

• Order-preserving maps for node inputs/output specs, and graph inputs/outputs. Without this, order is unstable as values are mutated in the editor.
• Graph evaluation should not panic when missing connections, as it crashes the editor when the preview is enabled. Tracking issue: Error handling during graph evaluation #22.