refactor(checkbox): implement setDisabledState method

Author: simeonoff

projects/igniteui-angular/src/lib/checkbox/checkbox.component.ts

@@ -454,6 +454,11 @@ export class IgxCheckboxComponent implements ControlValueAccessor, EditorProvide
         this._onTouchedCallback = fn;
     }
 
+    /** @hidden @internal */
+    public setDisabledState(isDisabled: boolean) {
+        this.disabled = isDisabled;
+    }
+
     /** @hidden @internal */
     public getEditElement() {
         return this.nativeCheckbox.nativeElement;