Greater customization of the LayerList widget

[RyanKelso]

It would be great to have more options and customization ability for the LayerList widget in Web Appbuilder.  Primarily, it would be great if you could add an assortment of map services or individual map service layers in the configuration of the widget, rather than having the widget simply inherit the layers that are present in the ArcGIS Online map that is "assigned" to the app.  It would allow greater control of which layers are included and which aren't, and you could set up multiple configurations of the LayerList widget in a single app to help organize the information for different users or purposes.