Restore serialization to use interfaces again

Chaning the fields to serializable classes breaks the existing
serializations.

Author: uhafner

src/main/java/edu/hm/hafner/util/FilteredLog.java

@@ -31,9 +31,7 @@ public class FilteredLog implements Serializable {
     private final int maxLines;
     private int lines;
 
-    @SuppressWarnings("PMD.LooseCoupling")
     private final ArrayList<String> infoMessages = new ArrayList<>();
-    @SuppressWarnings("PMD.LooseCoupling")
     private final ArrayList<String> errorMessages = new ArrayList<>();
 
     private transient ReentrantLock lock = new ReentrantLock();