Adds support for utf paths on Windows.

Not all file formats/calls are supported yet. It will be expended.

Please from now on use BLI_fopen, BLI_* for file manipulations.
For non-windows systems BLI_fopen just calls fopen.
For Windows, the utf-8 string is translated to utf-16 string in order to call UTF version of the function.

source/blender/imbuf/intern/png.c

@@ -195,7 +195,7 @@ int imb_savepng(struct ImBuf *ibuf, const char *name, int flags)
 		                 Flush);
 	}
 	else {
-		fp = fopen(name, "wb");
+		fp = BLI_fopen(name, "wb");
 		if (!fp) {
 			png_destroy_write_struct(&png_ptr, &info_ptr);
 			MEM_freeN(pixels);