Simplification via {{render-portal}} helper

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I think there is an way to simplify "pulling" declarative content into implementation.

It should be able to change the RenderPortal component mixin into a helper:

{{render-portal declarations className itemIndex itemSubindex}}

Here declarations are the list of PortalDeclaration collected. className is name of subclass of PortalDeclaration to filter by, itemIndex the # of the delcaration, and itemSubindex the index of the subclass of this.element presented by the PortalDeclaration instance.

For nested declarations, this could be used as:

{{render-portal declarations class1 item1 sub1 class2 item2 sub2 [etc as necessary] }}

In many situations, I'm hoping that this would eliminate the need for any render/... components. Of course, sometimes you might want them anyway (dynamic styling can't be guessed in declaration context perhaps?), but I think often they are just boiler plate.

I plan to try this out on a branch and use in ember-smenu ... if it looks good there we can try to re-base ember-grid on ember-declarative.