Add online (aka ReplayCenter) module

Add localization extra (formally known as LocalizedResourcePack)
Add version checker extra

src/main/java/com/replaymod/online/api/AuthData.java

@@ -0,0 +1,45 @@
+package com.replaymod.online.api;
+
+/**
+ * Represents a set of persistent authentication data.
+ */
+public interface AuthData {
+    /**
+     * Returns the user name of the authenticated user.
+     * @return user name or {@code null} if not logged in
+     */
+    String getUserName();
+
+    /**
+     * Returns the authentication key of the authenticated user.
+     * @return auth key or {@code null} if not logged in
+     */
+    String getAuthKey();
+
+    /**
+     * Store the authentication data after login.
+     * @param userName The user name
+     * @param authKey The authentication key
+     */
+    void setData(String userName, String authKey);
+
+    /**
+     * Result of authentication operations.
+     */
+    enum AuthResult {
+        /**
+         * The operation succeeded without errors.
+         */
+        SUCCESS,
+
+        /**
+         * The data provided got rejected due to it being invalid.
+         */
+        INVALID_DATA,
+
+        /**
+         * Operation could not be performed due to connectivity problems.
+         */
+        IO_ERROR,
+    }
+}