Conditional observers in bsn!

[hxYuki]

Objective

• Conditional observers in bsn!
• We have run_if() for observer systems, but IntoObserverSystem doesn't accept ObserverWithCondition which prevents adding conditions over observer in bsn!. And ObserverWithCondition is not cloneable, thus cannot be used in scene.

Solution

• Add a ConditionalOnTemplate, which holds the condition(s) and call Observer::run_if on build_template.
• Connect new condition with and_eager() when ConditionalOnTemplate::run_if called.
• Create ConditionalOnTemplate from OnTemplate via OnTemplate::run_if.

Testing

• Tested with on(|_event: On<PointerPress>| println!("Ok pressed!")).run_if(|| false) locally.

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Showcase

bsn! {
    @button("Ok")
    on(|_event: On<PointerPress>| println!("Ok pressed!")).run_if(|| false)
}