chore(*): add _filterIconsRegistered property

Author: ddincheva

projects/igniteui-angular/src/lib/grids/filtering/grid-filtering.service.ts

@@ -46,6 +46,7 @@ export class IgxFilteringService implements OnDestroy {
     private columnToExpressionsMap = new Map<string, ExpressionUI[]>();
     private _datePipe: IgxDatePipeComponent;
     private columnStartIndex = -1;
+    private _filterIconsRegistered = false;
     private _componentOverlayId: string;
     private _filterMenuPositionSettings: PositionSettings;
     private _filterMenuOverlaySettings: OverlaySettings;