Display actual language names in preferences

Add ability to set metadata variables in the language files.
Use the LANGUAGE_DISPLAY_NAME metadata variable value in preferences to
show the languages.

Metadata variables are defined in the first section of a language file,
i.e. the comments at the beginning of the file. At most one variable can
be defined per line. The definition takes the following format:
    #<header><whitespace>@<key>=<value>
where:
    <header> is any sequence of non-whitespace characters
    <whitespace> is any non-empty sequence of whitespace characters
    <key> is the metadata key name
    <value> is the metadata value
For example,
    #101 @LANGUAGE_DISPLAY_NAME=English (US)
defines a metadata key "LANGUAGE_DISPLAY_NAME" with value
"English (US)". Lines that do not match the format are ignored.

rtgui/multilangmgr.h

@@ -24,6 +24,25 @@
 
 #include <glibmm/ustring.h>
 
+class TranslationMetadata
+{
+public:
+    TranslationMetadata() = default;
+    ~TranslationMetadata() = default;
+    TranslationMetadata(const TranslationMetadata &other) = delete;
+    TranslationMetadata(TranslationMetadata &&other) = delete;
+    explicit TranslationMetadata(std::map<std::string, std::string> &&metadata);
+
+    TranslationMetadata &operator =(const TranslationMetadata &other) = delete;
+    TranslationMetadata &operator =(TranslationMetadata &&other) noexcept = default;
+
+    std::string get(const std::string &key, const std::string &default_value) const;
+    std::string getLanguageName(const std::string &default_name) const;
+
+private:
+    std::map<std::string, std::string> metadata;
+};
+
 class MultiLangMgr
 {
 public:
@@ -31,11 +50,13 @@ public:
 
     void load(const Glib::ustring &language, const std::vector<Glib::ustring> &fnames);
     Glib::ustring getStr(const std::string& key) const;
+    const TranslationMetadata *getMetadata(const Glib::ustring &fname) const;
     static bool isOSLanguageDetectSupported();
     static Glib::ustring getOSUserLanguage();
 
 private:
     std::map<std::string, Glib::ustring> translations;
+    mutable std::map<Glib::ustring, TranslationMetadata> lang_files_metadata;
 };
 
 extern MultiLangMgr langMgr;