Fix for WICKET-4852

Modified FeedbackPanel as per comments of the issue.
Modified default css files to reflect this changes.
Fix for WICKET-4852

Reverted CSS class changes
Tests for WICKET-4852

Added unit test with expected output
Tests for WICKET-4852

Apache License Header was missing
WICKET-4852 Do not set CSS class on the li > span element for a feedback message

Minor formatting improvements