Add system set to encompass systems in UiPlugin

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

The systems in `UiPlugin` are not all configured within one system set, which means it's not practical to order anything *before* those systems run. This is pertinent because UI-updating systems typically run in `Update`, so if you want any UI-controlling systems to run between UI updates and UI rendering, it is not feasible currently.

## What solution would you like?

Add system set in `PostUpdate` to encompass all systems in UiPlugin.

## What alternative(s) have you considered?

None

## Additional context

None

Locate the UiPlugin systems and inspect how they are scheduled in Update and PostUpdate. Add a system set in PostUpdate that encompasses all systems in UiPlugin, then verify that other systems can be ordered around the set between UI updates and rendering.

Written by the indexing model from the issue text.