[Feature Request] Option to display unchecked or checked checklist items only

[Selaron]

In contrast to #680 which might tend to alternate the manually applied order of checklist items, I'd like to have an option on top of the checklist which allows me to quickly toggle display of only checked or unchecked list items in task detail view. Text-Only items would stay visible regardless of selected option.

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While I think this would be quite useful for any long check list, my personal use case is that I have a Task representing a (quite long) shopping list where I uncheck things I need (soon) during the days and when shopping, I check things when put into the shopping cart. Seeing only those things to pick next would absolutely increase the overview and shopping experience. As I am a developer I think about providing a PR.