detached Radio(Group) memory leak

Steps to Reproduce

• Run the code

<zk>
    <div id="root">
        <div id="container">
            <radiogroup>
                <radio label="YES"/>
                <radio label="NO"/>
            </radiogroup>
        </div>
    </div>
    <button label="detach container" onClick="container.detach();"/>
</zk>

• click the button
• perform GC / take a heap dump
• check the live objects (e.g. in VisualVM)

Current Result

The <radio> component remains in memory even though it's been detached from the page

Expected Result

All detached components should be eligible for garbage collection.

Debug Information

from the heap dump it seems a callback from at the root div keeps a reference to the radio

the callback is added here:
https://github.com/zkoss/zk/blob/v9.5.0.2/zul/src/org/zkoss/zul/Radio.java#L289

Workaround