Add MucBookmarkAutojoinManager

Also add MucConfigFormManager and improve the MUC API (SMACK-648). Bump
to jxmpp 0.5.0-alpha3.

Improve and extend PrivateDataManager and BookmarkManager.

smack-extensions/src/main/java/org/jivesoftware/smackx/xdata/Form.java

@@ -353,6 +353,17 @@ public class Form {
         return dataForm.getField(variable);
     }
 
+    /**
+     * Check if a field with the given variable exists.
+     *
+     * @param variable the variable to check for.
+     * @return true if a field with the variable exists, false otherwise.
+     * @since 4.2
+     */
+    public boolean hasField(String variable) {
+        return dataForm.hasField(variable);
+    }
+
     /**
      * Returns the instructions that explain how to fill out the form and what the form is about.
      *