Generate events with WXK_NONE Unicode keys for non-characters in wxOSX.

Generate events with WXK_NONE Unicode keys for non-characters in wxOSX.

The keyboard keys not corresponding to real characters, such as cursor arrows
or function keys, must generate wxKeyEvents with WXK_NONE as Unicode key code
to make it possible to distinguish them from the printable characters but
wxOSX generated events with valid Unicode key codes for them instead.

Avoid this by excluding Unicode key codes corresponding to code points in the
Unicode private use area: while this doesn't seem to be documented anywhere,
all non-printable characters seem to have their Unicode representation inside
it.

This change brings wxOSX keyboard event generation in line with the other
ports and, as a side effect, also closes #12423.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68467 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775