Add ability to clean out tasks

[flamion]

Adds ability to "clean out" completed (or in progress) tasks.

Goal

Me and my friends utilize the mod to coordinate and keep tabs on what's still to do. Often there's a ton of old completed tasks piling up, I found myself wanting to "clean them up" before. Doing it one by one is a bit tedious so I had the idea to create a "clean" button that removes the tasks. The user is asked what should be cleaned and is then prompted with a confirmation dialog.

This might also close #82, depending on whether this suffices (as it's not automatic)

Here are a few screenshots:

Questions/Feedback request

I have tested this manually and did not add automated tests since I didn't know how exactly they work. It is also my first time doing something with Lua besides some stuff for Computercraft, so I might have made some mistakes or solved something in a suboptimal way.

[JonasJurczok]

Hello :)

Thanks for the contribution. I will take a look at the code and then merge it in. And yes, this should also solve #82 👍