Since [1] FDO notifications can transfer a `ActivateToken` to the client
application, we can use this token to set the correct platform data to
get wayland startup notification working correctly.
GNOME Shell gained the feature already in [2], so this is the last piece
missing to get rid of the annoying "<Application> is ready" notifications,
when clicking on a notification.

We can set the platform data only for actions that are activated via
`org.freedesktop.Application`.

Fixes: flatpak#406

[1] https://gitlab.freedesktop.org/xdg/xdg-specs/-/commit/b9a470004d
[2] https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3199/

Apps may not implement the `org.freedesktop.Application` interface so
they won't get the platform data. We can do a little better and add the
platform data to the `ActionInvoked` signal.