Use native support for grammar checking in NSTextView.
As we're not passing wxTextProofOptions to the lower level function,
this functionality now depends on wxUSE_SPELLCHECK, meaning that even
the previously existing MacCheckSpelling() function is not defined any
more when wxUSE_SPELLCHECK is set to 0. This is not completely
backwards-compatible, but hopefully shouldn't be a problem in practice
and shouldn't break any existing applications which can't disable the
just added wxUSE_SPELLCHECK.
db30397921