Fix wx[Sorted]ArrayString::Index when wxUSE_STL=1, because

  Fix wx[Sorted]ArrayString::Index when wxUSE_STL=1, because
it is different than wxArray::Index (second argument is for
case sensitivity, not search direction).
  Use forward declaration for wx[Sorted]ArrayString, where
possible.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@22128 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775