themes: Updated both dark and light themes menuitem prelight.

Updated both themes menuitem prelight to use same filename in gtkrc.
Added new ui/menuitem-prelight.png

themes/The-Dark-Side-of-Gimp/gtkrc

@@ -826,7 +826,7 @@ style "menu_item"
         {
           function = BOX
           state = PRELIGHT
-          file = "ui/entry-border-fill-active.png"
+          file = "ui/menuitem-prelight.png"
           border = { 1, 0, 1, 0 }
           stretch = TRUE
         }