CommandProcessor should post "long running action" notifications

[herzbube]

Currently every command implements its own handling for "long running action". This generally includes

• Implement a method that posts a notification and that can be invoked in the context of the main thread (because "long running action" notifications are guaranteed to be posted in the main thread context)
• Surround the entire doIt implementation with @try/@catch
• At the beginning of the @try block post the "action starts" notification
• In the @finally block post the "action ends" notification

It would be cleaner to implement this handling in CommandProcessor.

[herzbube]

A simple solution is to create a marker protocol (e.g. LongRunningAction) that can be adopted by commands that want to support "long running actions". When CommandProcessor detects the protocol it posts the start/end notifications before/after command execution begins/ends. Command execution is surrounded with @try/@catch.

One possible problem could be that some commands (LoadGameCommand?) may not execute all their parts entirely synchronously.