Allow setting Mac-specific menu item ids to wxID_NONE to suppress them.

Allow setting Mac-specific menu item ids to wxID_NONE to suppress them.

If s_macAboutMenuItemId or s_macPreferencesMenuItemId was explicitly set to
wxID_NONE by the application, don't add them to the standard menu at all,
otherwise they would be present there but be always disabled and useless.

This is still not documented because we really need a better API for this but
it at least provides a temporary workaround for having useless menu items
under OS X.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65560 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775