Add support to create Mixins

[aframevr]

Add support to create Mixins

[ngokevin]

Mixin creation through the editor is sort of scope creep, IMO. Mixins support not having to repeat code. Having them in the editor is sort of exposing code internals.

[fernandojsg]

The original idea was to create a simple modal with the list of components and attributes that you've modified, so you could click which components and attributes you would like to add to your mixin in a "blackbox" way.

What do you think? In any case if you don't like the possibility to add new mixins, would you also propose to remove the features to add a mixin to the entity or see the list of mixins already attached?

[ngokevin]

I guess it would help if you wanted to repeat a set of values across multiple entities. Though it adds a lot of complex UI and concepts. It's not something I would use since I'd rather code the mixin (and I'm working on mixin stylesheets), but I don't speak for everyone. Your call, just my input.

[ngokevin]

not a common workflow. mixins created from source