customElement as "templateController"

Custom elements should be able to be "template controllers". What I mean is the following:

Currently, to create a custom element that uses it's light DOM (child nodes) as a view/view factory, you have to jump through quite a few hoops. First, you need to add `@processContent(false)`, then, you need to inject `ViewResources, ViewSlot, ViewCompiler, Container, Element`, and lastly, in the constructor you need to do this whole circus:

``` js
const fragment = document.createDocumentFragment();

let child;

while ((child = element.firstElementChild) !== null) {
fragment.appendChild(child);
}

this.viewFactory = viewCompiler.compile(fragment, viewResources);
this.container = container;
this.viewSlot = viewSlot;
```

If this was a `templateController`, I'd be able to simply inject a `BoundViewFactory` and the `ViewSlot` and call it a day. My point is, I don't really care about the view resources, the container being used, or even the element in question. I just need all those things to go through a bunch of formalities to get the same sort of functionality that you can get for free on custom attributes, and I have to do it for every custom element I create that wants this kind of functionality.

Start by reading the existing custom-element and template-controller paths, especially the BoundViewFactory and ViewSlot usage described in the issue. Compare them with the manual ViewResources, ViewCompiler, Container, and Element setup. Done means a custom element can use its light DOM as a view or view factory without repeating that setup, with relevant behavior covered by the project’s existing tests.

Written by the indexing model from the issue text.