Fix button order in wxStdDialogButtonSizer in all cases under GTK.

The order of Yes/No/Cancel was recently fixed by the changes in r65346 but it
broke the order of the buttons in Ok/Cancel case. Ensure the correct order is
used in all cases now: the one described by GNOME HIG by default but different
order for the special case of Yes/No/Cancel which follows the native message
box.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65447 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775