title: PrimeNG calendar form group validation issue
body: <p>I'm trying to add validation to my form. </p>
  
  <p><code>discount</code> field cannot be empty and value range needs to be within 0 to 100, <code>time_from</code> and <code>time_to</code> cannot be empty. I cannot fire validation proccess on <code>time_from</code> and <code>time_to</code>. I use PrimeNG Calendar component. I found out that <code>p-calendar</code> validation works great with <code>ngModule</code>, but I couldnt find any solution for form groups.</p>
  
  <p><strong>Component (simplified)</strong></p>
  
  <pre><code>ngOnInit() {
      this.buildForm();
    }
  
    buildForm(): void {
      this.discountFG = this.fb.group({
        discount: new FormControl('', [Validators.required, CustomValidators.range([0, 100])]),
        time_from: new FormControl('', Validators.required),
        time_to: new FormControl('', Validators.required)
      });
  
      this.discountFG.valueChanges
        .subscribe(data =&gt; this.onValueChanged(data));
    }
  
    onValueChanged(data?: any) {
      if (!this.discountFG) { return; }
      const form = this.discountFG;
  
      for (const field in this.formErrors) {
        // clear previous error message (if any)
        this.formErrors[field] = '';
        const control = form.get(field);
  
        if (control &amp;&amp; control.dirty &amp;&amp; !control.valid) {
          const messages = this.validationMessages[field];
          for (const key in control.errors) {
            this.formErrors[field] += messages[key] + ' ';
          }
        }
      }
    }
  </code></pre>
  
  <p><strong>Template (simplified)</strong></p>
  
  <pre><code>          &lt;p-calendar formControlname="time_from" [locale]="pl" dateFormat="yy-mm-dd" [monthNavigator]="true" [yearNavigator]="true"
              yearRange="2010:2030" (blur)="setTimeFrom($event)" readonlyInput="true" required&gt;&lt;/p-calendar&gt;
  
            &lt;p-calendar formControlname="time_to" [locale]="pl" dateFormat="yy-mm-dd" [monthNavigator]="true" [yearNavigator]="true"
              yearRange="2010:2030" [minDate]="minDate" readonlyInput="true" required&gt;&lt;/p-calendar&gt;
  </code></pre>
  
  <p><strong>Current behavior</strong></p>
  
  <p>Validators dont notice if date is picked and because of this, no event is fired to catch value change, which means <code>onValueChanged</code> thinks <code>time_from</code> and <code>time_to</code> is untouched. </p>
  
  <p>How can I fix this ?</p>