The OK button of a dialog can have duplicate callback handlers

Dialogs that present a navigator component (tree + listing) will register a new callback handler on the OK button every time a different folder or document is selected.

Reproduction:

• select a document with Chrome
• open the copy dialog
• Inspect the OK button in the developer tools (right-click on it)
• open the 'Event Listeners' tab
• Make sure 'Framework listeners' checkbox is checked
• Open the click event
• It should contain one callback handler with source 'wicket-ajax-jquery.min-ver-xxxxxx.js'
• Now select a different folder in the tree component of the dialog
• Press the refresh button in the 'Event Listeners' tab
• Notice that there are now two event handlers with source 'wicket-ajax-jquery.min-ver-xxxxxx.js'

A different way to observe this behaviour is simply checking the 'Network' tab of the developer tools. After selecting a couple of folders in the dialog, a click on OK should trigger multiple callbacks.

Expected: only a single callback.