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.
288a301b97