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/unix/private/uilocale.h

@@ -0,0 +1,33 @@
+///////////////////////////////////////////////////////////////////////////////
+// Name:        wx/unix/private/uilocale.h
+// Purpose:     Various locale-related helpers used under Unix systems only
+// Author:      Vadim Zeitlin
+// Created:     2021-08-14 (extracted from src/common/intl.cpp)
+// Copyright:   (c) 2021 Vadim Zeitlin <vadim@wxwidgets.org>
+// Licence:     wxWindows licence
+///////////////////////////////////////////////////////////////////////////////
+
+#ifndef _WX_UNIX_PRIVATE_UILOCALE_H_
+#define _WX_UNIX_PRIVATE_UILOCALE_H_
+
+#include "wx/string.h"
+
+// get just the language part ("en" in "en_GB")
+inline wxString ExtractLang(const wxString& langFull)
+{
+    return langFull.BeforeFirst('_');
+}
+
+// get everything else (including the leading '_')
+inline wxString ExtractNotLang(const wxString& langFull)
+{
+    size_t pos = langFull.find('_');
+    if ( pos != wxString::npos )
+        return langFull.substr(pos);
+    else
+        return wxString();
+}
+
+const char *wxSetlocaleTryAll(int c, const wxString& lc);
+
+#endif // _WX_UNIX_PRIVATE_UILOCALE_H_