Add id:/needs: task dependencies

[GuilianCD]

Why

Tasks piling up overwhelm me, especially when some of them are not doable yet because they depend on another task being done first. In the spirit of GTD's "write things down to clear your mind", I wanted tuxedo to recognize task dependencies and change their appearance accordingly, i.e. dim them, or remove them from view.

I did not find a relevant extension of todo.txt that matched my needs, either.

What

This PR uses id:/needs: keys to keep track of what task needs which one. A task is blocked if it has a needs: field whose value exists in the value of another not-completed task's id: field.

Tasks that are blocked are dimmed.
Also added a B keybind (and palette/settings entry) that hides/shows blocked tasks.

I'm not certain about using id: but I don't know what else would fit.
The wording might be kind of naive and not necessarily super intuitive, particularly "blocked".