WICKET-6947 IndicatingAjaxButton does not work with Bootstrap 4

Non-functional changes.
Minor cleanup
Fix a typo in javadoc

Signed-off-by: Martin Tzvetanov Grigorov <mgrigorov@apache.org>
(cherry picked from commit 42f9372b242528796b93bf3e16d6ba0a35bdb7cd)
diff --git a/wicket-extensions/src/main/java/org/apache/wicket/extensions/ajax/markup/html/AjaxIndicatorAppender.java b/wicket-extensions/src/main/java/org/apache/wicket/extensions/ajax/markup/html/AjaxIndicatorAppender.java
index 770ca19..3a5c2e5 100644
--- a/wicket-extensions/src/main/java/org/apache/wicket/extensions/ajax/markup/html/AjaxIndicatorAppender.java
+++ b/wicket-extensions/src/main/java/org/apache/wicket/extensions/ajax/markup/html/AjaxIndicatorAppender.java
@@ -44,16 +44,13 @@
  */
 public class AjaxIndicatorAppender extends Behavior
 {
+	private static final long serialVersionUID = 1L;
+
 	/**
 	 * Component instance this behavior is bound to
 	 */
 	private Component component;
 
-	/**
-	 * 
-	 */
-	private static final long serialVersionUID = 1L;
-
 	private final ResourceReference indicator;
 
 	/**
@@ -65,7 +62,7 @@
 	}
 	
 	/**
-	 * Build a new instance specifing a custom busy indicator as {@link ResourceReference}.
+	 * Build a new instance specifying a custom busy indicator as {@link ResourceReference}.
 	 * 
 	 * @param indicator
 	 *             The {@link ResourceReference} to use as busy indicator.     
@@ -135,9 +132,6 @@
 		return component.getMarkupId() + "--ajax-indicator";
 	}
 
-	/**
-	 * @see org.apache.wicket.behavior.Behavior#bind(org.apache.wicket.Component)
-	 */
 	@Override
 	public final void bind(final Component component)
 	{
diff --git a/wicket-extensions/src/main/java/org/apache/wicket/extensions/ajax/markup/html/IndicatingAjaxLink.java b/wicket-extensions/src/main/java/org/apache/wicket/extensions/ajax/markup/html/IndicatingAjaxLink.java
index c428937..02aa57e 100644
--- a/wicket-extensions/src/main/java/org/apache/wicket/extensions/ajax/markup/html/IndicatingAjaxLink.java
+++ b/wicket-extensions/src/main/java/org/apache/wicket/extensions/ajax/markup/html/IndicatingAjaxLink.java
@@ -60,9 +60,6 @@
 		add(indicatorAppender);
 	}
 
-	/**
-	 * @see org.apache.wicket.ajax.IAjaxIndicatorAware#getAjaxIndicatorMarkupId()
-	 */
 	@Override
 	public String getAjaxIndicatorMarkupId()
 	{