Don't corrupt pasted UTF-16 text in wxOSX

Fix breakage introduced by 587067bc68:
pasting text could mangle some Unicode characters if it was in UTF-16.
The code in that commit converted newlines all right, but it also
converted _any_ occurrence of 0x0d to 0x0a, which wasn't very Unicode
friendly.

Fixed by partially reverting the change and adding a dedicated UTF-16
variant of wxMacConvertNewlines13To10() instead.

src/osx/core/cfstring.cpp

@@ -34,9 +34,9 @@ void wxMacConvertNewlines13To10( char * data )
     }
 }
 
-void wxMacConvertNewlines13To10( char * data, size_t len )
+void wxMacConvertNewlines13To10( wxChar16 * data )
 {
-    for ( ; len; ++data, --len )
+    for ( ; *data; ++data )
     {
         if ( *data == 0x0d )
             *data = 0x0a;