InventoryPlugin improvement for "waiting_for_activation" phase

Current Situation
The processing of the InventoryModel starts at the OnPrepare stage of a JobScheduler start. The InventoryModel then checks the state of the JobScheduler and starts processing only if the JobScheduler reaches the running stage. This is unnecessary, because if started at the onActivate stage no check for the JobScheduler state is needed, because at this stage the JobScheduler is already "running" for sure.

Desired Behavior
The Processing of the InventoryModel has to be moved to the OnActivate method of the Plugin class so that no more checking of the JobScheduler state is needed any further.