New action - UI - to provide interactive visualizer for tracked result

[seniorsolt]

UI provides the ability to fold and unfold tree elements and a convenient way to look at variables. To avoid making changes to "trace," I use a class attribute "events" to accumulate events tracked through trace, and a class method start() to explicitly start the UI:

import hunter
hunter.trace(action=hunter.actions.UI)

import os
os.path.join('a', 'b')
hunter.stop()
hunter.actions.UI.start()

For the implementation of the UI, I used PyQt5, so the style does not conform to PEP and uses CamelCase for overriding PyQt5 class methods.

I would like to request your review and some feedback.

I'm also unclear about the order of actions when I request to merge. Should I write additional tests, or is it enough to simply run tox on what already exists? Do I need to ensure test coverage in all environments specified in tox.ini?