Deprecated use of string values for inline compound nodes

### Feature request

Inline nodes extending `InlineCompoundNode` should not be accessed via `getValue` but `getChildren` should be used in the templates. 

```
    {%- for child in node.children -%}
        {{- renderNode(child) -}}
    {%- endfor -%}
```

We will remove the backward compatibility layer in version 2. 