sys: Tons of long overdue cleanup

- std::string -> const std::string& where needed
- Added a FileIO abstraction class
- Fixed recent files not updating
- Removed localization file from global include
- Renamed lang to pattern_language/pl
- Renamed EventFileDropped to RequestFileOpen

plugins/builtin/source/content/settings_entries.cpp

@@ -1,6 +1,9 @@
 #include <hex/api/content_registry.hpp>
 #include <hex/api/imhex_api.hpp>
 
+#include <hex/helpers/lang.hpp>
+using namespace hex::lang_literals;
+
 #include <imgui.h>
 
 #include <nlohmann/json.hpp>