Refactor ObjectList implementation

[whyscream]

Problem

The current Objectlist (the object that wraps a collection of API results, f.i. "get payments") is a messy class.

• It supports some methods that make it behave as a list, but also some dictionary-like behaviors.
• Iterating over paginated results is something that a developer must do in code, it is not provided by the client.
• It is initialized using a complex data structure (a full API response) which makes the code handling this data hard to read

Improvements

• Consume only the list data from the API response (Also see the TODO in code:

  mollie-api-python/mollie/api/objects/list.py

  Lines 17 to 19 in 5b63136

  	# TODO: it would be better if the ObjectList was initiated with a list of results, rather than with
  	# the full datastructure as it is now, so we can remove all this mucking around with fake data,
  	# mocked result objects, and loads of lengthy accessor workarounds everywhere in the ObjectList.

  )
• Automatic iteration over paginated API results
• Expose only list behavior

In #285 a POC is available. This should be properly integrated and tested.