Mixins included in selector/selectorAll schema property type results

Description:

I've been caught by this before, and I've had it come up for super-hands users (wmurphyrd/aframe-super-hands-component#116, possibly also wmurphyrd/aframe-super-hands-component#60).

Mixins can be returned from the query selection performed with the selector type schema properties, and any code expecting an entity and trying to call entity methods will cause errors. Attribute selectors can be particularly likely to cause this issue (but they are incredibly useful in A-Frame to select by component).

If there aren't any use cases where receiving mixins in the result is an expected behavior, then perhaps adding a step to filter mixins (or modifying the selector to exclude mixins) before passing the result on to the component could save a lot of head scratching.

If you're interested, I could prepare a PR

• A-Frame Version: all
• Platform / Device: all

Trace the selector and selectorAll schema-property query path, focusing on attribute selectors and how mixins enter the results. Reproduce the behavior with a mixin and an entity, then verify that the component receives entities without breaking valid selector results.