TButtonEdit: implement properties CanUndo, CaretPos, SelText.


git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@44652 4005530d-fff6-0310-9dd1-cebe43e6787f
TButtonEdit: make SetFocus set focus to the edit.


git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@44653 4005530d-fff6-0310-9dd1-cebe43e6787f