Create a general scheduling and timing system

[ianb]

We have timers and paused routines, and it's easy to imagine scheduled tasks, multiple timers, timers interacting with paused routines, etc.

To support this I think we should have a general system.

• Event types (like timers and pauses) would be subclasses from an abstract base class
• Events should be serializable and possible to restore, so they can persist over restarts
• Some queries over events should be possible
• Possibly there could be an event provider of sorts, that could read from an external calendar or something
• Events would generally have names
• Intents around timers and alarms should instantiate these events, or fetch them, but generally the intents will have minimal logic
• Events can be periodic (like "every day at 8am")
• When events are restored, and the event happened while the browser was closed, we should be careful about how that is handled (it's hard to know if it should always be ignored or not, without thinking through the specific cases)