title: Primeng p-dropdown Is filtering done according to the group name?
body: <p>I want to filter by group name using codes on PrimeNG page.</p>
  
  <pre><code>&lt;h3&gt;Group&lt;/h3&gt;
  &lt;p-dropdown [options]="groupedCars" [(ngModel)]="selectedCar3" placeholder="Select a Car" [group]="true" filter="true" filterBy="label,value"&gt;
      &lt;ng-template let-group pTemplate="group"&gt;
          &lt;img src="assets/showcase/images/demo/flag/{{group.value}}" style="width:20px;vertical-align:middle" /&gt;
          &lt;span style="margin-left:.25em"&gt;{{group.label}}&lt;/span&gt;
      &lt;/ng-template&gt;
  &lt;/p-dropdown&gt;
  &lt;p&gt;Selected Car: {{selectedCar3 || 'none'}}&lt;/p&gt;
  </code></pre>
  
  <p>I used groupedCars variable .ts page </p>
  
  <pre><code>selectedCar3: string;
  this.groupedCars = [
              {
                  label: 'Germany', value: 'germany.png', 
                  items: [
                      {label: 'Audi', value: 'Audi'},
                      {label: 'BMW', value: 'BMW'},
                      {label: 'Mercedes', value: 'Mercedes'}
                  ]
              },
              {
                  label: 'USA', value: 'usa.png', 
                  items: [
                      {label: 'Cadillac', value: 'Cadillac'},
                      {label: 'Ford', value: 'Ford'},
                      {label: 'GMC', value: 'GMC'}
                  ]
              },
              {
                  label: 'Japan', value: 'japan.png', 
                  items: [
                      {label: 'Honda', value: 'Honda'},
                      {label: 'Mazda', value: 'Mazda'},
                      {label: 'Toyota', value: 'Toyota'}
                  ]
              }
          ];
  </code></pre>
  
  <p>But, I want to filter By group labels and item labels. Is it possible?</p>