Clean up the API client

[kiddinn]

There are few things that need to be changed in the API client:

• [ ] Make all objects behave in a similar manner, eg how Aggregation works, where you can create an object and then configure it, and load it from a dict or a store, and then save it, which saves changes or creates a new one
• [ ] Add typing hints to all functions (update docstrings to py3)
• [ ] Fix the confusion about labels, SQL label vs. event label
• [ ] Improve explore in sketch, support more of the features in the ui, time ranges, label mixing, with AND or OR between them in addition to the query string.