Introduce a typedef for frontend handles.

This is another major source of unexplained 'void *' parameters
throughout the code.

In particular, the currently unused testback.c actually gave the wrong
pointer type to its internal store of the frontend handle - it cast
the input void * to a Terminal *, from which it got implicitly cast
back again when calling from_backend, and nobody noticed. Now it uses
the right type internally as well as externally.

unix/gtkmain.c

@@ -549,7 +549,7 @@ int do_cmdline(int argc, char **argv, int do_everything, Conf *conf)
     return err;
 }
 
-GtkWidget *make_gtk_toplevel_window(void *frontend)
+GtkWidget *make_gtk_toplevel_window(Frontend *frontend)
 {
     return gtk_window_new(GTK_WINDOW_TOPLEVEL);
 }