UnregisterReminder(string reminderName)

[alrz]

Right now we should keep a cached handle to unregister reminders. There's two problems with this:

• The cached instance may be null in the next activation.
• GetReminder may lead to an exception if the reminder is already unregistered.

If those are not considered misusage on my part, I think an idempotent helper that takes the reminder's name could simplify this boilerplate code that afaik is needed for every reminder.