LOCALE_IO rework to fix issues on linux when using env vars with non ASCII7 chars.

We need to use a "C" locate to read/write files, and therefore switch the locale.
Removing use of setlocale( LC_NUMERIC, "C" ) and use only
wxLocale( "C", "C", "C", false ) was too fast: on linux it breaks the env vars.
on MSW
    using setlocale( LC_NUMERIC, "C" ) generates an alert message in debug mode,
    and this message ("Decimal separator mismatch") must be disabled.
    But calling wxLocale( "C", "C", "C", false ) works fine
On unix:
    calling wxLocale( "C", "C", "C", false ) breaks env vars containing non ASCII7 chars.
    these env vars return a empty string from wxGetEnv() in many cases, and if such a
    var must be read after calling wxLocale( "C", "C", "C", false ), it looks like empty

So use wxLocale on Windows and setlocale on unix

common/locale_io.cpp

@@ -1,7 +1,7 @@
 /*
  * This program source code file is part of KiCad, a free EDA CAD application.
  *
- * Copyright (C) 1992-2020 KiCad Developers, see AUTHORS.txt for contributors.
+ * Copyright (C) 1992-2021 KiCad Developers, see AUTHORS.txt for contributors.
  *
  * This program is free software; you can redistribute it and/or
  * modify it under the terms of the GNU General Public License
@@ -24,15 +24,74 @@
 #include <locale_io.h>
 #include <wx/intl.h>
 
-std::atomic<unsigned int> LOCALE_IO::m_c_count( 0 );
+// When reading/writing files, we need to swtich to setlocale( LC_NUMERIC, "C" ).
+// Works fine to read/write files with floating point numbers.
+// We can call setlocale( LC_NUMERIC, "C" ) or wxLocale( "C", "C", "C", false )
+// wxWidgets discourage a direct call to setlocale
+// However, for us, calling wxLocale( "C", "C", "C", false ) has a unwanted effect:
+// The I18N translations are no longer active, because the English dictionary is selected.
+// To read files, this is not a major issues, but the resul can differ
+// from using setlocale(xx, "C").
+// Previouly, we used only setlocale( LC_NUMERIC, "C" )
+//
+// Known issues are
+// on MSW
+//    using setlocale( LC_NUMERIC, "C" ) generates an alert message in debug mode,
+//    and this message ("Decimal separator mismatch") must be disabled.
+//    But calling wxLocale( "C", "C", "C", false ) works fine
+// On unix:
+//    calling wxLocale( "C", "C", "C", false ) breaks env vars containing non ASCII7 chars.
+//    these env vars return a empty string from wxGetEnv() in many cases, and if such a
+//    var must be read after calling wxLocale( "C", "C", "C", false ), it looks like empty
+//
+// So use wxLocale on Windows and setlocale on unix
 
 
+// set USE_WXLOCALE 0 to use setlocale, 1 to use wxLocale:
+#if defined( _WIN32 )
+#define USE_WXLOCALE 1
+#else
+#define USE_WXLOCALE 0
+#endif
+
+// On Windows, when using setlocale, a wx alert is generated
+// in some cases (reading a bitmap for instance)
+// So we disable alerts during the time a file is read or written
+#if !USE_WXLOCALE
+#if defined( _WIN32 ) && defined( DEBUG )
+// a wxAssertHandler_t function to filter wxWidgets alert messages when reading/writing a file
+// when switching the locale to LC_NUMERIC, "C"
+// It is used in class LOCALE_IO to hide a useless (in kicad) wxWidgets alert message
+void KiAssertFilter( const wxString &file, int line,
+                     const wxString &func, const wxString &cond,
+                     const wxString &msg)
+{
+    if( !msg.Contains( "Decimal separator mismatch" ) )
+        wxTheApp->OnAssertFailure( file.c_str(), line, func.c_str(), cond.c_str(), msg.c_str() );
+}
+#endif
+#endif
+
+std::atomic<unsigned int> LOCALE_IO::m_c_count( 0 );
+
 LOCALE_IO::LOCALE_IO() : m_wxLocale( nullptr )
 {
     // use thread safe, atomic operation
     if( m_c_count++ == 0 )
     {
-        m_wxLocale = new wxLocale( "C", "C", "C", false );
+#if USE_WXLOCALE
+        #define C_LANG "C"
+        m_wxLocale = new wxLocale( C_LANG, C_LANG, C_LANG, false );
+#else
+        // Store the user locale name, to restore this locale later, in dtor
+        m_user_locale = setlocale( LC_NUMERIC, nullptr );
+#if defined( _WIN32 ) && defined( DEBUG )
+        // Disable wxWidgets alerts
+        wxSetAssertHandler( KiAssertFilter );
+#endif
+        // Switch the locale to C locale, to read/write files with fp numbers
+        setlocale( LC_NUMERIC, "C" );
+#endif
     }
 }
 
@@ -42,7 +101,16 @@ LOCALE_IO::~LOCALE_IO()
     // use thread safe, atomic operation
     if( --m_c_count == 0 )
     {
-        delete m_wxLocale; // Deleting m_wxLocale restored previous locale
+        // revert to the user locale
+#if USE_WXLOCALE
+        delete m_wxLocale;      // Deleting m_wxLocale restored previous locale
         m_wxLocale = nullptr;
+#else
+        setlocale( LC_NUMERIC, m_user_locale.c_str() );
+#if defined( _WIN32 ) && defined( DEBUG )
+        // Enable wxWidgets alerts
+        wxSetDefaultAssertHandler();
+#endif
+#endif
     }
-}
+}