Use correct extensions in wxDynamicLibrary::CanonicalizeName() on OS X.

Contrary to the documentation, this function incorrectly appended the
.bundle extension on OS X for libraries as well as modules.

Fixed to use .dylib for libraries (wxDL_LIBRARY) and continue to use
.bundle for loadable modules (wxDL_MODULE). Change GetDllExt() to take optional wxDynamicLibearyCategory argument.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74161 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775