Bug 57510: `Del' key does not delete on Mac; r=db48x, sr=jag

suite/common/mac/platformCommunicatorOverlay.xul

@@ -35,7 +35,10 @@
   <key id="key_selectAll" key="&selectAllCmd.key;" modifiers="accel"/>
 
   <!-- Delete Key -->
+  <!-- not all Mac keyboards have a VK_DELETE key, so we use VK_BACK as
+       the primary and provide VK_DELETE as a secondary key definition -->
   <key id="key_delete"  keycode="VK_BACK"   command="cmd_delete"/>
+  <key id="key_delete2" keycode="VK_DELETE" command="cmd_delete"/>
 
   <!-- This adds the Print Setup item to both Browser and Composer's
        Print toolbar button popup