[8u40] API proposal: [CSS, Control] Improve handling of Control.getUserAgentStylesheet()

[David Grieve]

Relative to https://javafx-jira.kenai.com/browse/RT-38640, I propose to 
add the following method to Region:

     /**
      * An implementation may specify its own user-agent styles for this 
Region, and its children,
      * by overriding this method. These styles are used in addition to 
whatever user-agent stylesheets
      * are in use. This provides a mechanism for third parties to 
introduce styles for custom controls.
      * @return A string URL
      */
     public string getUAStylesheet()

Note: an alternative is to make Control method getUserAgentStylesheet 
public and move it to Region. But I'm not sure about binary 
compatibility and, in the interest of time, I'm putting this API out 
there. The reasons for putting this API on Region is eloquently stated 
by Jonathan in the JIRA issue.