Teach the Edit Task modal to edit the 'On Completion' property

[claremacrae]

⚠️ Please check that this feature request hasn't been suggested before.

• I searched previous Ideas in Discussions didn't find any similar feature requests.
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• I am only requesting a single feature. Multiple changes should be split into individual requests, with links between them.
• I believe my requested feature will be generally applicable to other users, not just me: it is not uniquely tied to my personal workflow.

🔖 Feature description

The documentation for On Completion currently says:

This is to track that request.

✔️ Solution

Add an option in the Edit Task modal to allow setting and clearing the On Completion field.

Initially, it only needs to support:

obsidian-tasks/src/Task/OnCompletion.ts

Lines 4 to 8 in 30e7f0f

	export enum OnCompletion {
	Ignore = '',
	Keep = 'keep', // Like Ignore, but is visible on task lines
	Delete = 'delete',
	}

Users likely don't care about the difference between Ignore and Keep - so probably just offer one of them...

So perhaps the options would be:

What to do with this task when it is completed?
- Keep it [applies `Ignore`] - the default
- Delete it [applies `Delete`]

Possible design

There will eventually be other options supplied... So eventually more options will be needed in the modal.
It doesn't matter whether the first design is specific to the current options, or is obviously extensible to more options later on...

One user on Discord suggested putting the option here, after Start:

I had presumed it would go in the Status section at the bottom.

Another option is to put it near Recurs as I suggest that the On Completion facility is perhaps most used with Recurring tasks...

❓ Alternatives

For now, the only other option is to use Auto Suggest.

📝 Additional Context

No response