Bug 158065 [DataBinding] ValidationError observables need a nudge to hook themselves up
See attached patch.
bundles/org.eclipse.jface.databinding/src/org/eclipse/jface/databinding/observable/value/ComputedValue.java
IObservable[] newDependencies = ObservableTracker.runAndMonitor(
					privateInterface, privateInterface, null);
			stale = false;
			for (int i = 0; i < newDependencies.length; i++) {
				IObservable observable = newDependencies[i];
				
				if (observable.isStale()) {
					stale = true;
				} else {
					observable.addStaleListener(privateInterface);
				}
			}
			dependencies = newDependencies;
			dirty = false;
		}
		return cachedValue;
	}
	/**
	 * Subclasses must override this method to provide the object's value.
	 * 
	 * @return the object's value
	 */
	protected abstract Object calculate();
	protected final void makeDirty() {
		if (!dirty) {
			dirty = true;