restore wx 2.8 compat, improve string processing
Fix building with wx 2.8 by rewriting some more string related code. Replace all calls to .c_str() with .mb_str(). Remove some of the .c_str()/.mb_str() calls where the target is already wxString. Move the split()/enum_idx() functions from opts.cpp into str_split()/vec_find() in strutils.h/strutils.cpp for use in other files. Replace the C-style string parsing code in a couple of places in wxvbam.cpp for processing possible command line options by splitting on '='. Also replace a couple of places that use pointer arithmetic in widgets/joyedit.cpp and widgets/keyedit.cpp with wxString methods.