Refactor message handlers into injectable components (preparation for #1467)

src/main/java/fr/xephi/authme/message/MessagesFileHandler.java

@@ -0,0 +1,23 @@
+package fr.xephi.authme.message;
+
+import javax.inject.Inject;
+
+/**
+ * File handler for the messages_xx.yml resource.
+ */
+public class MessagesFileHandler extends AbstractMessageFileHandler {
+
+    @Inject // Trigger injection in the superclass
+    MessagesFileHandler() {
+    }
+
+    @Override
+    protected String createFilePath(String language) {
+        return "messages/messages_" + language + ".yml";
+    }
+
+    @Override
+    protected String getUpdateCommand() {
+        return "/authme messages";
+    }
+}