Fix stock accelerators handling in wxOSX.

Fix stock accelerators handling in wxOSX.

No modifiers were used for stock accelerators, i.e. Append(wxID_NEW) resulted
in an item with the label "New" and plain "N" as accelerator in wxOSX. This
was due to wxACCEL_CMD not being handled in wxAcceleratorEntry::ToString() so
map it to the same prefix as wxACCEL_CTRL there, this is enough to make the
accelerators work even though it's not clear whether this is really the best
thing to do or if we should use a separate "Cmd+" prefix for it (and recognize
it in Parse() too then) as otherwise it's unclear why do we have wxACCEL_CMD
at all, it just seems to be treated identically to wxACCEL_CTRL everywhere.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68463 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775