reuse sun.ComboBox's list to implement joist.PhetMenu

[pixelzoom]

Will need to factor out the list and add support for separators. ListNode will live in sun.

[samreid]

In my opinion, the popup PhetMenu should share code with other dialogs such as in #166. Many of the same issues apply, such as appearing over everything (z-order), appearing at the right location and updating location when the window bounds change. Perhaps we should look into this then?

[pixelzoom]

This functionality doesn't belong in joist. It will (someday) be handled more generally in phetsims/sun#347 ("Create a pane system for pop ups and dialogs").

Closing.