I need a Column in which it was possible to put a Widget. I have this:
import com.google.gwt.cell.client.AbstractCell;
import com.google.gwt.safehtml.shared.Safe
I have a post on adding a widget to a cell. Have a look and see if it helps you out.
http://mpickell.com/blog/2013/01/28/widgets-in-gwt-cell-tables/
Like Colin says, changes on the widget won't be used unless the HTML of the widget is pushed again to the page. I have some notes in the comments of this post on some ways you might handle events as well. Read the comments to see how to use the class I have in the post.
And also like Colin says here, be careful and make sure you understand what you're doing and how the widget is never actually attached to its HTML in the page.
By the way.. You mention you want a button. Why not use the GWT ButtonCell?