Pass the current timestamp as platform data when activating
an action. This is implemented slightly hacky, since we use
clutter_get_current_event_time() to get at the timestamp, but
the alternative is to expose g_action_muxer_activate_action_full
to js, which would be quite a bit more involved.
GDBusActionGroup api has changed again, adapt to that.
Also, use a GActionMuxer to add the 'app.' prefix to actions,
instead of manually stripping it out of the action names.
In the future, the muxer will also contain per-window actions
with a 'win.' prefix.
