WICKET-5420 Make UploadProgressBar easier to extend

Always remove the custom attribute Wicket sets to form element when calling its onsubmit() method.
onsubmit() may throw an exception and the attribute won't be removed.
WICKET-5420 Make UploadProgressBar easier to extend

Allow to set the value of the 'display' property with Wicket.DOM.show()
WICKET-5420 Make UploadProgressBar easier to extend

Fix a typo in comments
WICKET-5420 Make UploadProgressBar easier to extend
WICKET-5420 Make UploadProgressBar easier to extend

Change the type of the bar and status components to MarkupContainer so they can have children if needed