Support setting "split" cbs and properties

[ahyangyi]

Yeah... I know the implementation looks like a bit WET. Feel free to reorganize the things a bit. Needed that interface though.

[ldpl]

Looking at this PR, there are many more things to consider, like:

• Not allowing to split callbacks that don't have both parts:

  callbacks = {
  'graphics': 0xE0,
  'purchase_text': {
      'default': 1234,
      'purchase': 5678,
  }
  }

• Handling graphics/purchase graphics in the same way.

• Accessing and changing callbacks via object properties. I.e. train.callbacks.properties.cargo_capacity.default = ...

I need some time to think how to handle it the best and will probably end up implementing it myself.

[ahyangyi]

Yeah, and I think the need for object properties could also help eliminate the code smell that is the passing dictionaries everywhere (and having different behaviors depending on types).

[ldpl]

Superseded by b388165e8e0860e1f6bd44902a32606e4a0c73b3 Look into https://github.com/citymania-org/grf-py/blob/main/tests/test_lib_callback_manager.py for usage examples.