sum_tree: Do not implement Dimension on tuples, use new Dimensions wrapper instead (#35482)

This is a bit of a readability improvement IMHO; I often find myself
confused when dealing when dimension pairs, as there's no easy way to
jump to the implementation of a dimension for tuples to remind myself
for the n-th time how exactly that impl works. Now it should be possible
to jump directly to that impl.

Another bonus is that Dimension supports 3-ary tuples as well - by using
a () as a default value of a 3rd dimension.


Release Notes:

- N/A

Cargo.lock

@@ -3562,6 +3562,7 @@ dependencies = [
  "serde",
  "serde_json",
  "settings",
+ "sum_tree",
  "task",
  "theme",
  "ui",