Discussion: Can we make facets updateable?

In my daily worklife as developer, i often face cases, where i would just like to update a facet.

Example:
```

Some dynamic text


..... many content...

```

I case you would just like to update the text, based on a variable which could change every request, you need to update the whole panel.
If you use PrimeFaces, we also often use `p:autoUpdate` (which would be a nice addition to the JSF anyway!).

Actually i would like to:
```

Some dynamic text

```

or

```

```

I didnt thought yet about how to implement it but i dont think its impossible. Maybe we need a `#encodeFacet` or something in the Renderer.

WDYT @BalusC @arjantijms @melloware?

Start by reviewing the Renderer entry point mentioned in the discussion, including the possible #encodeFacet hook, and compare the p:autoUpdate and f:ajax render="pnl:@facet(header)" examples. Done should be defined as a supported way to update only a named facet without updating the whole panel.

Written by the indexing model from the issue text.