edit-runtime: sort non-null fields first

[oliver-sanders]

Sort fields in edit-runtime primarily by null/non-null status, and secondarily by alphabetical order or whatever.

Problem

Most of the time task definitions use only a small subset of the available configurations.

Most of the time when we edit-runtime, we are changing something which was already set.

So when we edit-runtime, we end up scrolling around trying to find the small subset of configurations which have actually been set.

Proposed Solution

Sort the fields in edit-runtime so that the fields which have been set appear first, then the unset fields bellow.

It might be worth considering displaying unset fields differently (until something is added into them) to make the distinction clear (to help explain the sort order).