[8u20] API Review: RT-35260 add user-agent stylesheets to Scene

[David Grieve]

This is coming a bit late in the game, but the proposal is to add this 
API to Scene:

     public ObservableList<String> getUserAgentStylesheets()

This would allow a Scene to have a user-agent style distinct from the 
application.

https://javafx-jira.kenai.com/browse/RT-35260