use separators to make Edit > Select menu more readable

gtk2_ardour/ardour.menus.in

@@ -163,9 +163,11 @@
         <menuitem action='select-all-tracks'/>
         <menuitem action='deselect-all'/>
         <menuitem action='invert-selection'/>
+        <separator/>
 	<menuitem action='select-loop-range'/>
 	<menuitem action='select-punch-range'/>
 	<menuitem action='select-from-regions'/>
+        <separator/>
 	<menuitem action='select-all-after-edit-cursor'/>
         <menuitem action='select-all-before-edit-cursor'/>
         <menuitem action='select-all-between-cursors'/>