`validateEmbedding` - a function that allows you to control what elements can be embedded to what other elements when the paper is put into the `embeddingMode` (see above). The function signature is: `function(childView, parentView)` and should return `true` if the `childView` element can be embedded into the `parentView` element. By default, all elements can be embedded into all other elements (the function returns `true` no matter what).