wayland: Fix handling of activation-token in org.freedesktop.Application requests

wayland: Fix handling of activation-token in org.freedesktop.Application requests

At the moment, GTK applications search for "desktop-startup-id" in the
platform data on Wayland , but desktop environments such as plasma set
"activation-token" property instead as indicated in the spec:

    activation-token: This should be a string of the same value as would
    be stored in the XDG_ACTIVATION_TOKEN environment variable, as specified
    by the XDG Activation protocol for Wayland.

https://specifications.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html#dbus