title: required validation does not work with kendo-combobox in angular
body: <p>I am using Kendo combobox with angular 5 , I am wondering if there is a way to apply some validations on this control, I have tried to use <code>required</code> attribute but it did not work, since kendo-combobox does not have property such as <code>.valid</code> to check if it is valid or not </p>
  
  <pre><code> &lt;kendo-combobox required #nameInput   [data]="availableCompanies" textField="displayText" valueField="value" [(value)]="company.selectedItem" 
                  [filterable]="true"  
                  [allowCustom]="true" [valueNormalizer]="valueNormalizer"  &gt;
                &lt;/kendo-combobox&gt;
  </code></pre>