title: SAPUI5 Aggregation for sap.m.Bar issue
body: <p>in my dialog I want to include a custom header or subheader with a button. My view.xml looks like this:</p>
  
  <pre><code> &lt;mvc:FragmentDefinition
     xmlns:mvc="sap.ui.core" xmlns:m="sap.m" xmlns:layout="sap.ui.layout" 
     xmlns:form="sap.ui.layout.form"&gt;
    &lt;m:Dialog id="dialog3ID" title="{i18n&gt;Deliveries}" contentHeight="auto"&gt;
        &lt;m:customHeader&gt;
          &lt;m:Bar&gt;
          &lt;contentLeft&gt;
               &lt;m:Button id="Btn_Back" iconDensityAware="false" text="
               {i18n&gt;Back}" type="Emphasized" width="10%" enabled="true" visible="true" press="onCancelD3" /&gt;
          &lt;/contentLeft&gt;
          &lt;/m:Bar&gt;
      &lt;/m:customHeader&gt;
      &lt;m:content&gt; 
        ......
      &lt;/m:content&gt;
  </code></pre>
  
  <p>The error message I get is Cannot add direct child without default aggregation defined for control sap.m.Bar</p>
  
  <p>What am I doing wrong?
  Thanks for your help,
  Tim</p>