When Wx is built with --enable-stl, wxString to wxChar*/char* implicit
conversions and vice-versa no longer work.

Change all wxChar* data members to wxString and change all pointer
arithmetic code (mostly in opts.cpp, cmdevents.cpp and
widgets/joyedit.cpp) to use wxString methods instead.

Also make mostly minor changes in various other files for all of this to
work.