Don't make wxQsort() extern "C" nor use any special convention for it.

It doesn't make sense to require passing C functions to wxQsort() that we
define in a C++ library. We also don't need any special calling convention
here.

Notice that the only existing uses of wxQsort() inside wxWidgets itself were
not actually extern "C" and one of them didn't even use the correct calling
convention.

Also avoid using non-wx-prefixed CMPFUNCDATA identifier in a public header,
rename it to wxSortCallback instead.

Finally make wxQsort() documentation slightly more useful.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69566 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775