make export dialog add button follow list selection

There are potential race conditions that are happening
between the dialog creation with empty list and the
thread that populates the list and fires selection change
events.

An easy hack is to make sure the button is enabled
whenever there are items in the list. This always
works, as the default behavior of list makes sure
that there is at least one item selected in a
non-empty list.

Fixes: #534

ui/org.eclipse.pde.ui/src/org/eclipse/pde/internal/ui/parts/ConditionalListSelectionDialog.java

@@ -67,6 +67,15 @@ public void setElements(Object[] elements) {
 		super.setElements(elements);
 		fElements = elements;
 	}
+
+	@Override
+	protected void updateOkState() {
+		super.updateOkState();
+		Button ok = getOkButton();
+		if (ok != null && !ok.isEnabled()) {
+			ok.setEnabled(fElements.length != 0);
+		}
+	}
 
 	public void setConditionalElements(Object[] elements) {
 		fConditionalElements = elements;