Make ObjectManager optional for Kernel::System modules

[bschmalhofer]

I see some use cases of using $Kernel::OM for object creation:

• providing singletons, potential better performance for objects that do not really hold data
• avoid overspill between different requests, as $Kernel::OM is destroyed after each request
• event registration and handling
• dependency checks
• registering constructor arguments for Get()
• registering default constructor arguments for Create() when the ObjectParam parameter is not defined

But for some modules, e.g. Kernel::System::DateTime these features are not useful, they produce only mental overhead. So we might want to ponder whether code like my $Now = Kernel::System::DateTime->new should be allowed. There is no technical difficulty here as the ObjectManager does the same thing when creating on object.

Currently we have the following declarations:

1. ObjectManagerDisabled : creation only with new()
2. IsSingleton: creation only with $Kernel::OM->Get()
3. NonSingleton: creation only with $Kernel::OM->Create()
4. none of the above: either $Kernel::OM->Get() or $Kernel::OM->Create()

Kernel::System::DateTime is declared as NonSingleton.
Technically we could create objects with ǹew() in any of these four cases. But this is definitly wrong in case 2. and maybe confusing in the cases 3. and 4. .Thus the question is only about the interface. The difficulty is that there is no obvious way how to check for the NonSingleton flag in the my $Now = Kernel::System::DateTime->new case.

A hack might be to provide derived classes that have a declaration of ObjectManagerDisabled. Or invent a completely new namespace for classes that have real object orientation.