Got rid of langMap in opds_dumper.cpp

Language code to human friendly name translation is now done with the
help of the ICU library. It works if the line

```
-include $(LANGSRCDIR)/resfiles.mk
```

in the file `source/data/Makefile.in` of the icu4c dependency is not
commented out. Currently, the said line is commented out (along with
some other include's) by the `icu4c_custom_data.patch` patch of the
`kiwix-build` tool.

src/opds_dumper.cpp

@@ -84,22 +84,22 @@ BookData getBookData(const Library* library, const std::vector<std::string>& boo
   return bookData;
 }
 
-struct LangInfo {
-  std::string selfName, englishName;
-};
-
-std::map<std::string, LangInfo> langMap = {
-      {"eng", { "English",  "English"} },
-      {"fra", { "Français", "French"} },
-      {"rus", { "Русский",  "Russian"} },
-};
-
 std::string getLanguageSelfName(const std::string& lang) {
-  return langMap.at(lang).selfName;
+  const icu::Locale locale(lang.c_str());
+  icu::UnicodeString ustring;
+  locale.getDisplayLanguage(locale, ustring);
+  std::string result;
+  ustring.toUTF8String(result);
+  return result;
 };
 
 std::string getLanguageEnglishName(const std::string& lang) {
-  return langMap.at(lang).englishName;
+  const icu::Locale locale(lang.c_str());
+  icu::UnicodeString ustring;
+  locale.getDisplayLanguage(icu::Locale("en"), ustring);
+  std::string result;
+  ustring.toUTF8String(result);
+  return result;
 };
 
 } // unnamed namespace

test/server.cpp

@@ -1043,7 +1043,7 @@ TEST_F(LibraryServerTest, catalog_v2_languages)
     <content type="text">All entries in English.</content>
   </entry>
   <entry>
-    <title>Français</title>
+    <title>français</title>
     <dc:language>fra</dc:language>
     <link rel="subsection"
           href="/catalog/v2/entries?lang=fra"
@@ -1053,7 +1053,7 @@ TEST_F(LibraryServerTest, catalog_v2_languages)
     <content type="text">All entries in French.</content>
   </entry>
   <entry>
-    <title>Русский</title>
+    <title>русский</title>
     <dc:language>rus</dc:language>
     <link rel="subsection"
           href="/catalog/v2/entries?lang=rus"