make public background/midground/foreground constants

direct/src/gui/DirectGuiGlobals.py

@@ -61,7 +61,11 @@ ERASE = PGEntry.getErasePrefix()
 IMAGE_SORT_INDEX = 10
 GEOM_SORT_INDEX = 20
 TEXT_SORT_INDEX = 30
-FADE_SORT_INDEX = 100
+
+# Handy conventions for organizing top-level gui objects in loose buckets.
+BACKGROUND_SORT_INDEX = -100
+MIDGROUND_SORT_INDEX = 0
+FOREGROUND_SORT_INDEX = 100
 
 defaultFont = None
 defaultClickSound = None