Add wxUILocale::CompareStrings() function

This function allows comparing strings using the sort order of the
specified locale, represented by the new wxLocaleIdent class.

It is implemented using CompareStringEx()[1] under MSW and
NSString::compare:options:range:locale:[2] under macOS, generic
implementation for the other platforms is upcoming.

[1]: https://docs.microsoft.com/en-us/windows/win32/api/stringapiset/nf-stringapiset-comparestringex
[2]: https://developer.apple.com/documentation/foundation/nsstring/1414561-compare?language=objc

include/wx/localedefs.h

@@ -108,6 +108,63 @@ struct WXDLLIMPEXP_BASE wxLanguageInfo
     const char* TrySetLocale() const;
 };
 
+// ----------------------------------------------------------------------------
+// wxLocaleIdent: allows to fully identify a locale under all platforms
+// ----------------------------------------------------------------------------
+
+class WXDLLIMPEXP_BASE wxLocaleIdent
+{
+public:
+    // Leave language empty
+    wxLocaleIdent() { }
+
+    // Construct name from language
+    wxLocaleIdent(const wxString& language) : m_language(language) { }
+
+    // Set language
+    wxLocaleIdent& Language(const wxString& language)
+    {
+        m_language = language;
+        return *this;
+    }
+
+    // Set region
+    wxLocaleIdent& Region(const wxString& region)
+    {
+        m_region = region;
+        return *this;
+    }
+
+    // Set script
+    wxLocaleIdent& Script(const wxString& script)
+    {
+        m_script = script;
+        return *this;
+    }
+
+    // Set modifier
+    wxLocaleIdent& Modifier(const wxString& modifier)
+    {
+        m_modifier = modifier;
+        return *this;
+    }
+
+    // Construct platform dependent name
+    wxString GetName() const;
+
+    // Empty language represents user's default language
+    bool IsDefault() const
+    {
+        return m_language.empty();
+    }
+
+private:
+    wxString m_language;
+    wxString m_region;
+    wxString m_script;
+    wxString m_modifier;
+};
+
 #endif // wxUSE_INTL
 
 #endif // _WX_LOCALEDEFS_H_