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.

fuzzterm.c

@@ -97,6 +97,7 @@ int is_iconic(void *frontend) { return 0; }
 void get_window_pos(void *frontend, int *x, int *y) { *x = 0; *y = 0; }
 void get_window_pixels(void *frontend, int *x, int *y) { *x = 0; *y = 0; }
 char *get_window_title(void *frontend, int icon) { return "moo"; }
+int frontend_is_utf8(void *frontend) { return TRUE; }
 
 /* needed by timing.c */
 void timer_change_notify(unsigned long next) { }

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);