Output debug messages to the debugger rather than stderr in Win32

This patch changes the behavior of ```debug()``` function in Win32 environment. It outputs the messages through the Windows' built-in debugging environments rather than ```stderr```.
In Windows, ```stderr``` is having trouble handling unicode strings. This patch mainly aims to print Unicode paths correctly.

This patch doesn't affect non-Windows environment.

This alters the behavior of TagLib in Win32, so I want to ask Windows users whether it is acceptable.