UI camera flexibility regression

## Bevy version

bf1ca81779

## What you did

In bevy 0.7, it was possible to manipulate individually ui cameras such as [in this example](https://github.com/nicopap/ui-navigation/blob/30c828a465f9d4c440d0ce6e97051a5f7fafa425/examples/off_screen_focusables.rs).

## What went wrong

With changes made to the way ui is handled in #4745, such design is impossible to execute. Manipulating UI cameras can be useful for fancy game effects, and it would be a shame to lose this flexibility.