Make the "seats =" block not deletable.

If this block is removed, the puzzle cannot be completed. THis will make a good example users can be pointed to for a use example of setDeletable(boolean).

demos/plane/blocks.js

@@ -31,6 +31,7 @@ Blockly.Blocks['plane_set_seats'] = {
     this.appendValueInput('VALUE')
         .appendField(Plane.getMsg('Plane_setSeats'));
     this.setTooltip(Blockly.Msg.VARIABLES_SET_TOOLTIP);
+    this.setDeletable(false);
   }
 };