Move PluginMetadata::GetSimpleMessages() to ToSimpleMessages()

It's more versatile and would avoid duplication in LOOT's code.

include/loot/metadata/message.h

@@ -154,6 +154,19 @@ LOOT_API bool operator>=(const Message& lhs, const Message& rhs);
 LOOT_API std::optional<SimpleMessage> ToSimpleMessage(
     const Message& message,
     const std::string& language);
+
+/**
+ * Get the messages as SimpleMessages given a language.
+ * @param  language
+ *         The preferred language for the message content.
+ * @return A vector of SimpleMessage objects for the preferred language, or for
+ *         English if message text is not available for the given language. The
+ *         order of the input Message objects is preserved, though any messages
+ *         without the preferred language or English content will be omitted.
+ */
+LOOT_API std::vector<SimpleMessage> ToSimpleMessages(
+    const std::vector<Message>& message,
+    const std::string& language);
 }
 
 #endif

include/loot/metadata/plugin_metadata.h

@@ -144,15 +144,6 @@ public:
    */
   LOOT_API std::vector<Location> GetLocations() const;
 
-  /**
-   * Get the plugin's messages as SimpleMessage objects for the given language.
-   * @param language
-   *        The language to create the SimpleMessage objects for.
-   * @return The plugin's messages as SimpleMessage objects.
-   */
-  LOOT_API std::vector<SimpleMessage> GetSimpleMessages(
-      const std::string& language) const;
-
   /**
    * Set the plugin's group.
    * @param group