Add a gdb pretty-printer for wxFileName objects.

Add a gdb pretty-printer for wxFileName objects.

This pretty-printer works by calling the C++ wxFileName::GetFullPath()
directly instead of interpreting the object data from gdb because this is much
simpler to do, so it can only be used when debugging live programs. If this is
a serious restriction, it would need to be changed to use gdb data access only
later.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71303 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775