Be case insensitive when looking for language catalogs on Windows.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74252 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Fix GetMsgCatalogSubdirs() to do as documentation says.

Due to a typo, the prefix/lang path wasn't considered when opening a
domain catalog.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74253 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775