Add wxUSE_STD_CONTAINERS and turn it on by default.

Add wxUSE_STD_CONTAINERS and turn it on by default.

Previously, wxUSE_STL enabled both implicit conversion of wxString to
std::[w]string and use of standard containers for the implementation of their
wx equivalents. Split up the two roles now by allowing to enable the use of
the standard containers independently of (backwards incompatible) implicit
conversion in wxString and actually enable wxUSE_STD_CONTAINERS by default.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67343 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775