Passing common data only to some widgets.

I think that we should be able to have a common descriptor which only applies to i.e. 3 widgets, not all of them.

Imagine having these widgets in a single application:
```
um-widget1
um-widget2
um-widget3
tp-widget1
tp-widget2
tp-widget3
widget1
widget2
widget3
```

```

{
"baseUrl": "http://localhost:9090"
}

```

I would like to override this in a single place for all the widgets prefixed with `um`. I could either do
```

{
"umBaseUrl": "http://localhost:7080"
}

```
or override the `baseUrl` in every widget descriptor. I think that neither of these solutions is really clean. Instead, you should be able to specify `data-union-common="um"` in the common descriptor and something like `data-union-group="um"` in the widget descriptors. Perhaps we could allow comma-separated values in here as well.

Start by locating how the common descriptor and widget descriptors are discovered and matched. Trace how the existing data-union-common attributes are applied, then define the grouping behavior and verify that a common descriptor can target only the intended widgets without changing unrelated widgets.

Written by the indexing model from the issue text.