chore(*): Adding handling for scenario where column is initially pinned and is then unpinned and needs to go to the initial index of the columnsList.

Author: MKirova

projects/igniteui-angular/src/lib/grids/columns/column.component.ts

@@ -547,6 +547,7 @@ export class IgxColumnComponent implements AfterContentInit {
             /* No grid/width available at initialization. `initPinning` in the grid
                will re-init the group (if present)
             */
+            this._unpinnedIndex = this.grid.columns.filter(x => !x.pinned).indexOf(this);
             this._pinned = value;
             this.pinnedChange.emit(this._pinned);
         }