Deprecate w-extend and allow multiple widgets to be bound to the same HTML element

[mlrawlings]

There is the limitation that a component cannot w-bind to another component.

The following is not currently allowed:

<some-component w-bind onEvent="handleEvent"/>

You can however wrap it, but who wants that extra div?

<div w-bind>
    <some-component onEvent="handleEvent"/>
</div>

The alternative is to use w-extend, which looks like this:

<some-component w-extend onEvent="handleEvent"/>

This actually extends the component and merges it with your component. They share state, etc.
It's a bit confusing and we don't have any documentation for it, but the code lives here and it complicates things when rendering.

We want to add support for the first thing:

<some-component w-bind onEvent="handleEvent"/>

This means allowing a single dom element to be bound to multiple widgets.