Core : Ensure asynchronous calls are supported on probes

[Michael Janelle-Montcalm]

We do not have a strong synchronization mechanism for information sent via DBus from the Engine and from the Creator. The execution scenario is the following :

The engine enters a state. It needs to install all the actions, then install all the event filters. When proceeding like this in an asynchronous mode (sending all the install commands, then all the subscribe commands), there is no guarantee that all the actions will have been installed before handling the events. Also, we have no way to wait until everything has been properly installed in the activities before waiting for the callback.

We suggest that there should be a clean synchronization mechanism that will handle all those cases and provide the necessary guarantees to the Engine :

1 - When exiting the _setup_state method, all of the actions and all of the events have been installed.
2 - When doing a tearDown on a State, all the actions and event filters are removed before we can issue more orders.