added implementations of fl_wcwidth_() and fl_wcwidth() to src/fl_utf.c

these supercede the old fl_wcwidth() code in src/xutf8/fl_wcwidth.c.
also added corresponding declarations to FL/fl_utf.c, and updated
src/Fl_Text_Buffer.c to enable the call to fl_wcwidth()



git-svn-id: file:///fltk/svn/fltk/branches/branch-1.3@7551 ea41ed52-d2ee-0310-a9c1-e6b18d33e121