Forbid creation of wxStandardPaths object directly.

This happens to work under MSW and Unix where there is only one
wxStandardPaths class for both the console and the GUI applications but
doesn't return the correct result under OS X where the Core Foundation
version, returned by wxStandardPaths::Get(), has to be used for the GUI
programs. And historically this confused a lot of people, so just ensure that
they can't accidentally create an object of the wrong type any more.

Closes #13537.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74428 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775