refactor pomo to use countdown threads and events

[twitchett]

precursor to https://github.com/AndrumDev/rumplenator/issues/29

timer logic has changed as follows:

• a thread is spawned per work or break countdown instead of per pomo sesssion
• at the end of a countdown, either a new thread is spawned to start the next work/break timer, or the pomo_complete callback is invoked
• if the countdown is cancelled, an on_cancel handler is invoked
• each countdown thread has access to an Event object, which can be used to cancel from the main thread