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/uilocale.h

@@ -45,6 +45,10 @@ public:
     wxString GetInfo(wxLocaleInfo index,
                      wxLocaleCategory cat = wxLOCALE_CAT_DEFAULT) const;
 
+    // Compares two strings, for a locale specified by wxLocaleIdent.
+    static int CompareStrings(const wxString& lhs, const wxString& rhs,
+                              const wxLocaleIdent& localeId = wxLocaleIdent());
+
     // Note that this class is not supposed to be used polymorphically, hence
     // its dtor is not virtual.
     ~wxUILocale();