Make templates immutable components that apply themselves via hooks

[alice-i-cecile]

From https://discord.com/channels/691052431525675048/1318745402199113859/1318794634817962007

[JMS55]

The only issue with this is specifying when the templates get updated.

Presumably users would spawn an entity with a template component, and then there would be a system that each frames iterates over all template entities and re-runs the template build. However users probably want control over when those systems run.

Probably you could have a system for each of the built-in schedules (Update, PostUpdate, etc), and then have people parameterize their template component like struct AutoBuildingTemplate<S: ScheduleLabel> { template: Template }. Each system can then iterate only over the matching components.

For more manual control over scheduling and system ordering, users could add their own systems like they do now.

[JMS55]

The other issue actually is that AutoBuildingTemplate won't have access to any system params for reading ECS data.

Potentially instead of storing a Template directly, it should store Box<dyn Fn(&World) -> Template>, and the templates get updated in an exclusive system. That's how I've done it in bevy_dioxus and similar.