Issue #196: Use LayerCreator to fully support <layer/> in OververviewMap widget.
Issue #196: Clean up.
Issue #196: Set up main map & overview map handlers after both have loaded.
Issue #196: Fix overview map basemap layer being created with isOpLayer property set to true.
Issue #196: Simplify overview map graphics layer logic.
Merge branch '#196/support-layer-tag-in-overview-map-widget' into develop