API to clear and add Stream subscribers

[philippjfr]

Currently Streams can have subscribers and hidden subscribers. In working on some projects I have found utility in reusing an existing stream but clearing the subscribers manually and manually adding new subscribers since the _hidden_subscribers are not public API this is quite ugly. It would be nice if Streams offered methods to add new subscribers (.listen or .subscribe?) and clear existing subscribers (.clear?).

[jlstevens]

It would be reasonable to have an 'advanced user' API for such things that is public but not something most people would need to worry about most of the time. Definitely better than fiddling with underscore attributes!

From what I remember, _hidden_subscribers partially exists because there needs to be a callback to the plotting class instance. This means calling clear could break the normal stream examples if it removes all subscribers, including everything in _hidden_subscribers. This isn't to say clear is a bad idea, just that it is advanced API for people who really understand how the streams system works in detail.

[philippjfr]

Done in PR referenced above.

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