Rework XML Element hierarchy and XmlStringBuilder

- Reduce the amount of types that are subtypes of NamedElement. See
javadoc of NamedElement for rationale.
- Work more with XmlEnvironment in XmlStringBuilder.
- Some minor changes to XmlStringBuilder API.

smack-core/src/test/java/org/jivesoftware/smack/StanzaCollectorTest.java

@@ -188,5 +188,10 @@ public class StanzaCollectorTest {
         public String toString() {
             return toXML().toString();
         }
+
+        @Override
+        public String getElementName() {
+            return "packetId";
+        }
     }
 }

smack-core/src/test/java/org/jivesoftware/smack/util/PacketParserUtilsTest.java

@@ -738,10 +738,6 @@ public class PacketParserUtilsTest {
             .a("type", "chat")
             .a("xml:lang", "en")
             .e("body")
-                // TODO: Remove the following xml:lang once Smack's serialization toXml() API is aware of a potential
-                // scoping xml:lang value. The out message stanza already declares an xml:lang with the exact same
-                // value, hence this statement is redundant.
-                .a("xml:lang", "en")
                 .t("This is a test of the emergency broadcast system, 1.")
             .up()
             .e("body")