NavigationBarScreenButton rewrite so we don't have 8 copies of our icon per screen.  Significantly smaller scene graph for some sims. Possibly relates to #137, #108, #94, #80.
NavigationBarScreenButton rewrite so we don't have 8 copies of our icon per screen.  Significantly smaller scene graph for some sims. Possibly relates to #137, #108, #94, #80.

phetsims/joist,phetsims/joist,phetsims/joist