Allow to do ambiguous controller add()

[customlogic]

From our discussion in #53

Allow folders to do the ambiguous ".add" to create new controllers, appends it to itself, and returns the controller's own view

[mflux]

  folder.add(
    DATGUIVR.add( mesh.position, 'y', 2, 6 ).listen(),
    DATGUIVR.add( material, 'wireframe' ).listen(),
    DATGUIVR.add( animation, 'speed', {fast: 1, slow: 0.1 } ).listen()
  );

Will now be

  gui.add( mesh.position, 'y', 2, 6 ).listen();
  gui.add( material, 'wireframe' ).listen();
  gui.add( animation, 'speed', {fast: 1, slow: 0.1 } ).listen();

[mflux]

09115b2b7e2249cf357b729b8c1e39a3c8702b53

[mflux]

Tests need to be updated

[customlogic]

done in e4af9bd