Do not set CSS class on the li > span element for a feedback message

In a mail discussion http://markmail.org/thread/glqn5n6lneh5fkjr has been proposed:

Martin Grigorov:

• <div> element should have class "feedbackPanel" (this is already the case)
• <li> element(s) should have class that specifies the feedback
  message level (currently by default Wicket sets "feedbackPanelLEVEL",
  but this is configurable with
  org.apache.wicket.markup.html.panel.FeedbackPanel#getCSSClass(FeedbackMessage))
• the <span> should not have class at all (currently it has the same
  class as the <li> element)
• the styling should be done with CSS selectors (e.g.
  div.feedbackPanel; div.feedbackPanel li.alert-warn; div.feedbackPanel
  li.alert-warn span; ...)
• if custom markup is needed then a custom FeedbackPanel is needed
  (one that extends from the default FeedbackPanel or a completely new
  one, it depends on the use case)

Sven Meier:

To ease extending of FeedbackPanel I'd suggest to delegate #newItem() of the
nested MessageListView to a new protected method #newMessageItem() in
FeedbackPanel. (Similar to DataTable#newRowItem())

Since we cannot make these improvements for 6.x because it may break applications which depend on this behavior we will postpone them for Wicket 7.