notification persistence (sticky reminders)

[ajvsol]

That way the notification doesn't disappear if 'clear' all notifications is accidentally clicked, so now it requires snooze or marked done to disappear.

[CampelloManuel]

also requested by another user in #478

refer to https://github.com/tasks/tasks/commit/dd48d074bea72d39e9a6721d442611b8500e5ae4 for how to implement

We have to choose how to make the reminders sticky

• 1 app-wide setting: either they're all sticky, or not
• 2 app-wide settings, to make only lists of "tasks" or "notes" have sticky notifications
• a setting for each list, note or reminder. These would require a new column (isSticky) on their respective tables in the database

I think the note-wide setting is the best: add a toggle-button near the due date to force all reminders to show sticky notifications. I'm open to suggestions on this.

UPDATE: Notification.java and its table have a permanent field, so it would be easier to add an option to declare a reminder as permanent, and that will propagate on how we show the notification