Trail enhancements for ComboBox

[pixelzoom]

So that I can move forward with phetsims/sun#430 (comment) ...

(1) Please add visibility annotations for Trail fields

(2) Please add containsNode to Trail. Probably some thing like this (untested):

/**
 * Does this Trail contain the specified Node?
 * @param {Node} node
 */
containsNode( node ) {
  return event.trail.nodes.indexOf( node ) !== -1;
}

[pixelzoom]

If trail.nodes was intended to be @private, then we have problems. I see 39 occurrences of trail.nodes, several of which are in sim code. And most occurrences are testing to see if a Node is on the Trail.

E.g. fractions-common/BuildingLabScreenView:

const doesTrailMatch = _.includes( event.trail.nodes, screen.view ) || event.trail.length <= 1;

[pixelzoom]

If trail.nodes was intended to be public, then you can skip adding containsNode, and probably just document nodes as @public (read-only).

[jonathanolson]

Should be done above. Added containsNode anyways, as it will be more convenient.

Does this look good? (Sorry about the whitespace formatting in the commit)

[pixelzoom]

👍