shell: Fix resizing when switching from panel to panel

set_active_panel_from_id doesn't remove the active
panel from the notebook before adding a new one. After that the shell
window can't shrink below the size of the original panel.

This might also cause crashers as some parts of the panel are left
alive, behind another notebook tab, ready to receive signals.

https://bugzilla.gnome.org/show_bug.cgi?id=636596

shell/gnome-control-center.c

@@ -776,6 +776,8 @@ _shell_set_active_panel_from_id (CcShell      *shell,
     }
   else
     {
+      gtk_notebook_remove_page (GTK_NOTEBOOK (priv->notebook), CAPPLET_PAGE);
+
       activate_panel (GNOME_CONTROL_CENTER (shell), start_id, desktop, name,
                       icon_name);