widgets: Don't force selection of GimpRow

As seen in the Welcome Dialog, if you have multi-selection
in place for a ListBox and try to Ctrl-click another row,
the call to gtk_list_box_select_row () will deselect the
prior row unless you click on an area that's not covered
by GimpRow. This patch removes the forced selection,
with the assumption that the click will bubble through
as normal to GtkListBox.

app/widgets/gimprow.c

@@ -382,9 +382,6 @@ gimp_row_button_press_event (GtkWidget      *widget,
 
       if (GTK_IS_LIST_BOX (list_box))
         {
-          gtk_list_box_select_row (GTK_LIST_BOX (list_box),
-                                   GTK_LIST_BOX_ROW (widget));
-
           if (context_menu)
             {
               GtkWidget *editor = gtk_widget_get_ancestor (widget,