[assistant-stock-split] set parent window properly

without this commit, the assistant-stock-split window will be hidden
if the register is activated.

gnucash/gnome/assistant-stock-split.c

@@ -789,6 +789,7 @@ gnc_stock_split_dialog (GtkWidget *parent, Account * initial)
         return;
     }
 
+    gtk_window_set_transient_for (GTK_WINDOW (info->window), GTK_WINDOW(parent));
     gtk_widget_show_all (info->window);
 
     gnc_window_adjust_for_screen (GTK_WINDOW(info->window));

gnucash/gnome/gnc-plugin-page-register.c

@@ -4563,13 +4563,15 @@ gnc_plugin_page_register_cmd_stock_split (GtkAction* action,
                                           GncPluginPageRegister* page)
 {
     Account* account;
+    GtkWindow* window;
 
     ENTER ("(action %p, plugin_page %p)", action, page);
 
     g_return_if_fail (GNC_IS_PLUGIN_PAGE_REGISTER (page));
 
     account = gnc_plugin_page_register_get_account (page);
-    gnc_stock_split_dialog (NULL, account);
+    window = gnc_window_get_gtk_window (GNC_WINDOW (GNC_PLUGIN_PAGE (page)->window));
+    gnc_stock_split_dialog (GTK_WIDGET (window), account);
     LEAVE (" ");
 }