Make winapi_mutex_wrapper official

[gast128@…]

I couldn't find a reference in the documentation. The winapi_mutex_wrapper has some advantages over the named_mutex:

• + not file based; i.e. no chance of leaking files in case crash
• + no problem of singleton destruction order. We use 'remove' in our own singleton but that gives a problem being in 'atexit'
• - Win32 / Win64 specific

While portability is a good thing, we solely develop for Windows so the more handy Windows solution is ok. There is already a windows_shared_memory (which is also more handy than the portable shared_memory_object). Can't this one be promoted to official?