url-mode: add support for XDG activation when opening URLs

First, add a ‘token’ argument to spawn(). When non-NULL, spawn() will
set the ‘XDG_ACTIVATION_TOKEN’ environment variable in the forked
process. If DISPLAY is non-NULL, we also set DESKTOP_STARTUP_ID, for
compatibility with X11 applications. Note that failing to set either
of these environment variables are considered non-fatal - i.e. we
ignore failures.

Next, add a helper function, wayl_get_activation_token(), to generate
an XDG activation token, and call a user-provided callback when it’s
‘done (since token generation is asynchronous). This function takes an
optional ‘seat’ and ‘serial’ arguments - when both are non-NULL/zero,
we set the serial on the token. ‘win’ is a required argument, used to
set the surface on the token.

Re-write wayl_win_set_urgent() to use the new helper function.

Finally, rewrite activate_url() to first try to get an activation
token (and spawn the URL launcher in the token callback). If that
fails, or if we don’t have XDG activation support, spawn the URL
launcher immediately (like before this patch).

Closes #1058

spawn.h

@@ -5,7 +5,8 @@
 #include "reaper.h"
 
 bool spawn(struct reaper *reaper, const char *cwd, char *const argv[],
-           int stdin_fd, int stdout_fd, int stderr_fd);
+           int stdin_fd, int stdout_fd, int stderr_fd,
+           const char *xdg_activation_token);
 
 bool spawn_expand_template(
     const struct config_spawn_template *template,