Templating: Remove need for unnecessary dom transformations

[hannessolo]

We should think of a way to use slots in html templates to include existing html elements in the block in the template without needing to explicitly copy them over.

[dicagno]

@bdelacretaz makes sense to assign this to you considering your work on #48 ?

[bdelacretaz]

I think PR #47 indeed avoids extra transformations, but it's using a different templating mechanism with data-aem-selector attributes in templates, reducing the amount of JS code. At this point I see it more as a low-code alternative, but as @fnhipster mentioned elsewhere we might merge both approaches - later, I'd say.