Use the best instead of current page size in OS X preferences dialog.

The page, which is typically just a wxPanel, is usually not created with the
correct size, so resize it to its best size before showing it in the
preferences dialog.

This makes OS X behaviour consistent with MSW and GTK and is generally more
useful.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74041 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775