title: Blockly how to prevent concatenated blocks from splitting
body: <p>In my toolbox.xml I have created a custom block by concatenating multiple blocks, something like </p>
  
  <pre><code>&lt;block type="my_custom_type"&gt;
      &lt;value name="LIST_REQUIRED"&gt;
          &lt;block type="lists_create_with" inline="true"&gt;
              &lt;mutation items="3"&gt;&lt;/mutation&gt;
              &lt;value name="ADD0"&gt;
                  &lt;block type="get_variable"/&gt;
              &lt;/value&gt;
              &lt;value name="ADD1"&gt;
                  &lt;block type="get_variable"/&gt;
              &lt;/value&gt;
              &lt;value name="ADD2"&gt;
                  &lt;block type="get_variable"/&gt;
              &lt;/value&gt;
          &lt;/block&gt;
      &lt;/value&gt;
  &lt;/block&gt;
  </code></pre>
  
  <p>in this specific case, a list block is united with my_custom_type block.</p>
  
  <p>After an user drags this new block to the workspace, how do I prevent him from removing the "list" block from the original "my_custom_type" block? At the moment he can do that by clicking on the "list" block, which allows him to take that piece separately.</p>