I see FormButtonBinding in the 2.x API but not in the 3.x API. Nor do I see it in the legacy API. From the javadocs, it looks like this was meant to monitor the state of a form and the enabled/disabled state of all buttons.
You might take a look at FormPanel to see if it's usable for your situation. If not, it would be fairly trivial to create something that does the behavior you are seeking. Take a look at the TextField.*valid* methods inherited from the ValueBaseField and Field classes.
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You can combine GXT Fields with the GWT Editor framework.
You achieve binding with the editors, and validation with the fields.