Rethink flushing

[WhyNotHugo]

Flushing deletes all completed todos.

For my shopping list, this is fine, but for list of tasks, I prefer to keep a history, even if not indefinitely.

Maybe flush should explicitly take a collection as input?

It's also annoying that the sequential ids also count completed tasks. Most of my tasks are three digits, but I rarely have 100 pending tasks. Perhaps completed tasks should have a different ID sequence? This might be a problem since the ID changes on completion and undoing becomes non-trivial.

Leaving this issue open for future thoughts before working on this.