New SystemActivityMonitor.

- SystemActivityMonitor is an ActivityMonitor that is used internally to handle ActivityMonitor.LoggingErrors.
	- When configured with a LogPath, it writes any errors it receives in independent text files inside the directory.
	- It raises a secured event with the error message (and with the potential exception if it failed to write the error log inside LogPath).
	- It can be used as a standard ActivityMonitor, except that it contains a non removable internal IActivityMonitorClient and it is
	not configured by the ActivityMonitor.AutoConfiguration actions: it does does not have GrandOutputClient.

- Cleanup of the dead client (WeakRef) of GrandOutput is done.