Local UI stacks

## What problem does this solve or what need does it fill?

The UI stacking order is stored in the `UiStack` resource in a single ordered list of entities which makes its difficult to track local changes.

For example, if you add a new node to the center of the list, every node after it gets a new stack index. Even if the nodes aren't ordered against each other, for instance because they are rendered to different cameras.

## What solution would you like?

To enable coarser change tracking, split up UI stack into local lists stored on each root node

## What alternative(s) have you considered?

Some sort of tree of spans with dirty flags, maybe. Rebuilding each flat list isn't that expensive though, it may not be worth it.