Setting WM_CLASS is deprecated in GTK

[vain]

browser.c: In function ‘client_new’:
browser.c:51:5: warning: ‘gtk_window_set_wmclass’ is deprecated [-Wdeprecated-declarations]
     gtk_window_set_wmclass(GTK_WINDOW(c->win), window_name, window_class);
     ^~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/gtk-3.0/gtk/gtkdialog.h:32:0,
                 from /usr/include/gtk-3.0/gtk/gtkaboutdialog.h:30,
                 from /usr/include/gtk-3.0/gtk/gtk.h:31,
                 from browser.c:4:

They don't appear to suggest a replacement for this function. It is pretty important on X11, though, (and so is geometry handling), so I won't simply remove it.