title: Define fields outside and then bring into items lits
body: <p>I am using extJs 6. I have code like below, it can be a text box or anything else also.  I want to make a common repo for all the fields and then include in the reports by using xtype or something else, because same code will be used in multiple reports.</p>
  
  <pre><code>  items: [
                  {
                      xtype: 'combo',
                      fieldLabel: 'year',
                      name: 'Year',
                      displayField: 'Key',
                      valueField: 'Year',
                      value: new Date().getFullYear(),
                      store: {
                          type: 'years'
                      }
                  },
  </code></pre>
  
  <p>It should look like </p>
  
  <pre><code>   items: [
                      {
                          xtype: 'yearfield'
                      },
            ]
  </code></pre>
  
  <p>or </p>
  
  <pre><code>  items: [
                              xtype: 'yearfield'
                ]
  </code></pre>
  
  <p>As what should i define these fields and what should it extend.</p>