(v5) Overlay system

[Kingminer7]

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Most (if not all) mods that use keepAcrossScenes try to keep their node above the current scene, so why not just have an overlay to make that easier? I've added an overlay that gets visited after the CCScene, so it'll always render above and won't have weirdness moving the node between changing CCScenes.

I also have a fork of DevTools prepared which renders the overlay node below the CCScene.

Marked as draft as it contains breaking changes and should only be merged with geode v5.

Credit to Alphalaneous for the idea on how to implement this!