[API REVIEW] Add javafx.scene.layout.BackgroundInsets (RT-26277)

[David Grieve]

https://javafx-jira.kenai.com/browse/RT-26277 relates to the use of percentage values in -fx-background-insets. BackgroundFill (in javafx.scene.layout) uses Insets, which does not allow for a percentage value. 

My proposal here is to simply copy the BorderWidths implementation to a BackgroundInsets class, with appropriate renaming along the way, and use it in place of Insets in BackgroundFill. The impact on the BackgroundFill API is:

-    public final Insets getInsets() 
+    public final BackgroundInsets getBackgroundInsets() 

Since BackgroundFill is new in 8.0, backward compatibility is not a concern.