Hotloading actor is broken.

[Stan]

As mentioned on ​Phab:rP25210 hotloading actors has the following defects

• Can crash the game when just adding and removing props.
• Changing materials no longer has any effect.
• When using animated meshes, e.g a unit holding a sword and the sword is missing, instead of one warning you'll get a dozen each time it moves.

[wraitii]

In 25308:

Hotloading fix after rP25210

Emplace does not replace existing element, insert_or_assign does.

While at it:

• Clean up the 'outdated' logic and reuse it for actors
• When an actor fails to load, return a placeholder. This improves hotloading of broken actors, and makes Cunit behaviour more predictable.
• Some minor cleanup

Reported by: Stan

Fixes #6157

Differential Revision: ​https://code.wildfiregames.com/D3882