Fix item selection/focus drawing in generic wxListCtrl.

Fix item selection/focus drawing in generic wxListCtrl.

Item focus rectangle was not drawn at all under wxGTK as the code doing it was
disabled with a comment saying that it was drawn elsewhere -- but this wasn't
the case.

So remove #ifdefs for wxGTK/Mac from generic wxListCtrl code and do use
wxRendererNative methods for all platforms. This fixes the appearance of the
control under GTK and if it introduces any problems under Mac, they should be
fixed in its wxRendererNative implementation and not by adding #ifdefs here.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64879 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775