Rather than calling `.extendSchema()`, have `.updateSchema()` return the new schema fragment

Description:

Rather than having to know about and call extendSchema.

  AFRAME.registerComponent('example', {
    updateSchema: (data) {
      var newSchema;
      if (data.type !== this.data.type) {
        newSchema = getNewSchema(data.type);
        this.extendSchema(newSchema);
      }
    }
  });

Just return the new schema fragment. This will reduce the public API by a bit:

  AFRAME.registerComponent('example', {
    updateSchema: (data) {
      var newSchema;
      if (data.type !== this.data.type) {
        return getNewSchema(data.type);
      }
      return null;
    }
  });

Start at the AFRAME.registerComponent lifecycle and inspect how updateSchema and extendSchema are currently used. Trace the handling of the value returned by updateSchema, including the no-change case. Done means updateSchema can return a new schema fragment without requiring callers to invoke extendSchema directly, while existing component behavior remains intact.