Add an example of a shared sphere

[vincentfretin]

Add an example in the repo of a shared sphere where each participant can click on it to change its color.
see discussion https://aframevr.slack.com/archives/C65MC3NSF/p1611344560006300
with the glitch https://glitch.com/edit/#!/naf-material-color that use a sphere in the html with persistent:true

[vincentfretin]

Another example we could add #232 (comment)
A sphere with random color set by the first participant, the other participants see the same color. But the code was specific to the easyrtc adapter. The interesting code here was this:

NAF.connection.onConnect(() => {
  const scene = document.querySelector("a-scene");
  const room = scene.getAttribute("networked-scene")["room"];
  const initialOccupantsCount = Object.keys(NAF.connection.adapter.easyrtc.getRoomOccupantsAsMap(room)).filter((occupantId) => NAF.clientId !== occupantId).length;
  const mysphere = document.getElementById("mysphere");
  if (initialOccupantsCount === 0) {
    console.log("I'm the first participant");
    NAF.utils.takeOwnership(mysphere);
  }
});

[vincentfretin]

When doing this example, look at the #178 issue about the different ownership events.

[vincentfretin]

There is actually the http://localhost:8080/ownership-transfer.html example that demonstrate how to spawn a cube and take ownership of it. We may adapt it to add a raycaster component?

[vincentfretin]

see also #258

[vincentfretin]

When the creator (not owner) of a networked entity leaves the room, the entity is removed from the room. Using persistent:true disable the removal.
In the code, if using persistent:false and if the creator field is empty, it fallback to owner, so the entities with creator unknown are removed if the current owner leaves. The only case where creator is empty is when you're specifying the owner yourself, but not the creator. So actually you can have persistent by specifying persistent:false;creator:scene but this is confusing. :)

persistent:true;owner:scene in the html, specifying a non empty owner string that isn't a real participant, is used so that the user that join the room won't send this entity because not the current owner, the creator field is actually empty but we don't care because persistent:true.