Enhance wxUILocale and wxLocaleIdent

Many improvements and fixes to wxUILocale:

- Add wxUILocale method for retrieving wxLocaleIdent identifier,
  localized names, layout direction.
- Add wxLocaleIdent attributes, getter, and setter for
  platform-dependent tags under Windows: extension, sort order.
- Modify method wxLocaleIdent::FromTag to support not only BCP 47-like
  tags, but also platform-dependent syntax.
- Modify method wxLocaleIdent::GetTag to allow specifying the tag type.
- Update internat sample to better show using wxUILocale.
- Update German and French message catalogs for internat sample (German
  fully translated, French msgIds only).
- Introduced wxUILocaleImplStdC under Windows, because locale "en-US" is
  not equivalent to the C locale.
- Adjust wxLocale class to restore previous wxUILocale in the
  destructor.
- Implement wxLocale::GetInfo method through wxUILocale methods.
- Removed LCID dependency in wxLocale.
- Move the implementation of some static wxUILocale methods from
  intl.cpp to uilocale.cpp.

Co-authored-by: Vadim Zeitlin <vadim@wxwidgets.org>

Closes #2615.

include/wx/intl.h

@@ -270,6 +270,7 @@ private:
                    m_strShort;        // short name for the locale
     int            m_language;        // this locale wxLanguage value
 
+    wxString       m_oldUILocale;     // previous wxUILocale name
     const char  *m_pszOldLocale;      // previous locale from setlocale()
     wxLocale      *m_pOldLocale;      // previous wxLocale
 #ifdef __WIN32__