R1:1644b4505057

Disperse task subpriorities in blocks

Summary:
Ref T7664. The current algorithm for moving task subpriorities can end up stuck in a real sticky swamp in some unusual situations.

Instead, use an algorithm which works like this:

- When we notice two tasks are too close together, look at the area around those tasks (just a few paces).
- If things look pretty empty, we can just spread the tasks out a little bit.
- But, if things are still real crowded, take another look further.
- Keep doing that…