We now have GRemoteActionGroup interface with the needed API,
implemented by GDBusActionGroup.
With the new API we could theoretically turn GActionMuxer itself into a
GRemoteActionGroup and expose the _full API to the shell so that the
timestamps could be passed from there.
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.
