Fix: new Grease Pencil layer not selected when added from the viewport

When adding a new layer from the viewport, the newly created layer
is set as active, which is visible in the properties panel,
but the selection in the dopesheet was not updated accordingly,
due to a missing notifier which is added in this patch.

source/blender/editors/gpencil/gpencil_data.c

@@ -247,6 +247,7 @@ static int gpencil_layer_add_exec(bContext *C, wmOperator *op)
                       ID_RECALC_TRANSFORM | ID_RECALC_GEOMETRY | ID_RECALC_COPY_ON_WRITE);
   }
   WM_event_add_notifier(C, NC_GPENCIL | ND_DATA | NA_EDITED, NULL);
+  WM_event_add_notifier(C, NC_GPENCIL | ND_DATA | NA_SELECTED, NULL);
 
   return OPERATOR_FINISHED;
 }