Please describe the problem:
When a character is outside the WM charset (e.g. the Euro symbol when the WM is
in ISO-8859-1 locales), the WM_NAME property isn't modified by
gtk_set_window_title(). This is a problem with window managers like fvwm2 that
don't support _NET_WM_NAME. Non-representable characters should be replaced by a
"?" for instance.

Steps to reproduce:
1. Use ISO-8859-1 locales.
2. Start fvwm2.
3. Start Firefox.
4. Open a webpage with a Euro symbol, e.g.
<https://bugzilla.mozilla.org/attachment.cgi?id=202537>.

Actual results:
The window title after opening the page is still the old one; for instance,
xprop gives for the Firefox window:
WM_NAME(STRING) = "Bug 160236 - no window title if none of the *_NAME properties
are set - Mozilla Firefox"
_NET_WM_NAME(UTF8_STRING) = 0xe2, 0x82, 0xac, 0x20, 0x2d, 0x20, 0x4d, 0x6f,
0x7a, 0x69, 0x6c, 0x6c, 0x61, 0x20, 0x46, 0x69, 0x72, 0x65, 0x66, 0x6f, 0x78

Expected results:
WM_NAME should contain: "? - Mozilla Firefox".

Does this happen every time?
Yes.

Other information:
See <https://bugzilla.mozilla.org/show_bug.cgi?id=315947> where it has been
suggested (in comment #5) to report a bug here.