Create `handle_manifest` function

This method must retrieve from a `const map` the method/function to call for each `widgets`/`listeners` and tell the widget entrypoint to be called.
That must respond something like this :
```json
{
  "widgets": [
    "root",
    "home"
  ],
  "listeners": [
    "onEnvStart",
    "onUserFirstJoin",
    "onSessionStart"
  ],
  "rootWidget": "root"
}
```