Lua UI - Method to get all elements on a layer

There is currently no way for the Lua UI to return all (or any) elements given a layer name.

This would be useful for debugging and for cinematic mods or mods that require hiding all UI (or need to manipulate all UI).

For example, this debug mod I am working on has to be manually hooked into an element to debug it; but being able to query all elements in a layer would allow more efficient usage.

This is separate from the intentions of #8958