Make MarkableExtension a singleton

to reduce pressure on the memory subsystem.

smack-experimental/src/main/java/org/jivesoftware/smackx/chat_markers/ChatMarkersManager.java

@@ -125,7 +125,7 @@ public final class ChatMarkersManager extends Manager {
                     SmackException.NotLoggedInException {
                 Message message = (Message) packet;
                 // add a markable extension
-                message.addExtension(new ChatMarkersElements.MarkableExtension());
+                message.addExtension(ChatMarkersElements.MarkableExtension.INSTANCE);
             }
         }, OUTGOING_MESSAGE_FILTER);
 

smack-experimental/src/main/java/org/jivesoftware/smackx/chat_markers/element/ChatMarkersElements.java

@@ -41,14 +41,16 @@ public class ChatMarkersElements {
      * @author Fernando Ramirez
      *
      */
-    public static class MarkableExtension implements ExtensionElement {
+    public static final class MarkableExtension implements ExtensionElement {
+
+        public static final MarkableExtension INSTANCE = new MarkableExtension();
 
         /**
          * markable element.
          */
         public static final String ELEMENT = ChatMarkersState.markable.toString();
 
-        public MarkableExtension() {
+        private MarkableExtension() {
         }
 
         @Override

smack-experimental/src/main/java/org/jivesoftware/smackx/chat_markers/provider/MarkableProvider.java

@@ -34,7 +34,7 @@ public class MarkableProvider extends ExtensionElementProvider<MarkableExtension
 
     @Override
     public MarkableExtension parse(XmlPullParser parser, int initialDepth) throws Exception {
-        return new MarkableExtension();
+        return MarkableExtension.INSTANCE;
     }
 
 }

smack-experimental/src/test/java/org/jivesoftware/smackx/chat_markers/MarkableExtensionTest.java

@@ -41,7 +41,7 @@ public class MarkableExtensionTest {
         Message message = new Message(JidCreate.from("ingrichard@royalty.england.lit/throne"));
         message.setStanzaId("message-1");
         message.setBody("My lord, dispatch; read o'er these articles.");
-        message.addExtension(new ChatMarkersElements.MarkableExtension());
+        message.addExtension(ChatMarkersElements.MarkableExtension.INSTANCE);
         Assert.assertEquals(markableMessageStanza, message.toXML(null).toString());
     }