win_set_[icon_]title: send a codepage along with the string.

While fixing the previous commit I noticed that window titles don't
actually _work_ properly if you change the terminal character set,
because the text accumulated in the OSC string buffer is sent to the
TermWin as raw bytes, with no indication of what character set it
should interpret them as. You might get lucky if you happened to
choose the right charset (in particular, UTF-8 is a common default),
but if you change the charset half way through a run, then there's
certainly no way the frontend will know to interpret two window titles
sent before and after the change in two different charsets.

So, now win_set_title() and win_set_icon_title() both include a
codepage parameter along with the byte string, and it's up to them to
translate the provided window title from that encoding to whatever the
local window system expects to receive.

On Windows, that's wide-string Unicode, so we can just use the
existing dup_mb_to_wc utility function. But in GTK, it's UTF-8, so I
had to write an extra utility function to encode a wide string as
UTF-8.

misc.h

@@ -215,6 +215,11 @@ bool smemeq(const void *av, const void *bv, size_t len);
  * been removed. */
 size_t encode_utf8(void *output, unsigned long ch);
 
+/* Encode a wide-character string into UTF-8. Tolerates surrogates if
+ * sizeof(wchar_t) == 2, assuming that in that case the wide string is
+ * encoded in UTF-16. */
+char *encode_wide_string_as_utf8(const wchar_t *wstr);
+
 /* Write a string out in C string-literal format. */
 void write_c_string_literal(FILE *fp, ptrlen str);