Platforms now sends fine-grained execution progress.

Progress was previously communicated on the basis of execution sets;
the grain is now quite finer since the execution of every single
programs is notified (both when it begins or ends).

This is done with the help of:
- a new ProgramStatus.Type: WAITING,
- a new event that can be send by platforms to server:
  EXECUTION_PROGRESS
- an ExecutionProgressMonitor specially designed for these events.

This applies to every deploiement configurations, from simple,
standalone ones to configurations embedding the specific
PlatformToClientBridge (which allows local executions along with
distant ones).