Move platform-specific parts of wxLocale::Init() to wxUILocale

This is tidier than using #ifdefs in the same common file and also
ensures that initializing wxLocale affects wxUILocale too, which will be
important for compatibility when the code elsewhere is modified to use
wxUILocale::GetInfo() instead of wxLocale::GetInfo() in the upcoming
commits.

This commit is best viewed with --color-moved git option.

include/wx/msw/private/uilocale.h

@@ -0,0 +1,28 @@
+///////////////////////////////////////////////////////////////////////////////
+// Name:        wx/msw/private/uilocale.h
+// Purpose:     MSW-specific locale-related helpers
+// Author:      Vadim Zeitlin
+// Created:     2021-08-14
+// Copyright:   (c) 2021 Vadim Zeitlin <vadim@wxwidgets.org>
+// Licence:     wxWindows licence
+///////////////////////////////////////////////////////////////////////////////
+
+#ifndef _WX_MSW_PRIVATE_UILOCALE_H_
+#define _WX_MSW_PRIVATE_UILOCALE_H_
+
+#include "wx/msw/private.h"     // Include <windows.h> to get LCID.
+
+#ifndef LOCALE_SNAME
+#define LOCALE_SNAME 0x5c
+#endif
+#ifndef LOCALE_CUSTOM_UI_DEFAULT
+#define LOCALE_CUSTOM_UI_DEFAULT 0x1400
+#endif
+
+// Use the specific LCID for the current thread.
+void wxUseLCID(LCID lcid);
+
+// This function is defined in src/common/intl.cpp
+wxString wxGetInfoFromLCID(LCID lcid, wxLocaleInfo index, wxLocaleCategory cat);
+
+#endif // _WX_MSW_PRIVATE_UILOCALE_H_