For Bandbox, Combobox, and Combobutton, setOpen shouldn't depend on isVisible.

The test file B86-ZK-4054.zul requires the `setOpen` method in Bandbox.java, Combobox.java, and Combobutton.java to check for visibility (via `isVisible`) before setting the property `_open` as demonstrated in this commit

However, we believe that `setOpen` shouldn't depend on `isVisible`, and we intend to change it. Since there are users that depends on the old behavior, we elucidate this change.