Draft: notification: Get the activation-token form ActionInvoked when using portal
Since [1] the xdg-desktop-portal-gtk adds the activation-token to the ActionInvoked signal. We can store it and let apps use it like the activation-token we get from FDO.
[1] https://github.com/flatpak/xdg-desktop-portal-gtk/pull/468/
CC: @3v1n0