Add ability to interrupt other threads

[wmanley]

As long as they are blocked on stbt.frames() or time.sleep()

We add the ability to interrupt a thread that we've previously spawned - as long as they are blocked on get_frame or time.sleep. This takes care of tearing threads down once we're done with them without requiring the user to add any complicated logic for this purpose. This is the invasive part that requires changes to _stbt.core

This same mechanism could be used to implement a timeout contextmanager.

Example:

def look_for_logo():
    return wait_until(lambda: match("logo.png"))

t = threading.Thread(target=look_for_logo)
t.start()

interrupt(t)
t.join()

Notes

• This is broken out of RFC: Stbt.threading APIs #463. Hopefully it should be enough to be able to experiment with threading APIs outside of stbt before we bake something into it.
• https://stackoverflow.com/a/325528/65031 provides an alternative approach. Less invasive, but more likely to provoke bugs

TODO

• Merge Display: Allow multiple threads to call get_frame() and frames() #449
• Add test
• Close-up review

[drothlis]

Rebased onto master to get Travis fixes from #474.

[drothlis]

Somehow this breaks test_press_visualisation consistently, both on Travis and my laptop.

[wmanley]

See also: Timeouts and cancellation for humans