Support frontend_is_utf8() in all front ends.

Previously only Unix front ends bothered to include it, on the basis
that only the pty backend needed it (to set IUTF8 in the pty). We're
about to need it everywhere else too.

windows/window.c

@@ -229,6 +229,11 @@ void frontend_echoedit_update(void *frontend, int echo, int edit)
 {
 }
 
+int frontend_is_utf8(void *frontend)
+{
+    return ucsdata.line_codepage == CP_UTF8;
+}
+
 char *get_ttymode(void *frontend, const char *mode)
 {
     return term_get_ttymode(term, mode);