LCL: Cleanup TNotebook.SetPageIndex.

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@46468 4005530d-fff6-0310-9dd1-cebe43e6787f
Designer: Name a record better, TUndoList -> TUndoItem.

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@46469 4005530d-fff6-0310-9dd1-cebe43e6787f