TButtonEdit: implement methods Clear, ClearSelection, CopyToClipboard, CutToClipboard, PasteFromClipboard and Undo.
(Issue #0025991)


git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@44654 4005530d-fff6-0310-9dd1-cebe43e6787f