foot/client: implement xdga client activation

This is an application of the xdg activation protocol that will allow
compositors to associate new foot toplevels with the command that
launched them.

footclient receives an activation token from the launcher which the
compositor can use to track application startup. It passes the token
to the foot server, which then activates the new window with the token
to complete the startup sequence.

pgo/pgo.c

@@ -75,7 +75,7 @@ render_xcursor_set(struct seat *seat, struct terminal *term, const char *xcursor
 }
 
 struct wl_window *
-wayl_win_init(struct terminal *term)
+wayl_win_init(struct terminal *term, const char *token)
 {
     return NULL;
 }