Create handle_manifest function

[shiipou]

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 :

{
  "widgets": [
    "root",
    "home"
  ],
  "listeners": [
    "onEnvStart",
    "onUserFirstJoin",
    "onSessionStart"
  ],
  "rootWidget": "root"
}