UI Opacity

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

This allows fade-in fade-out transitions for UI bundles, and setting the overall transparency of UI bundles, which I think is useful in many games to unclutter the window when it's not really important (but may be needed) or as a configuration in games for players who wish to unclutter their view and see more what's going on.

## What solution would you like?

To be discussed.

## What alternative(s) have you considered?

For my own game, I am considering trying to do pop-in pop-out animation with [`Transform`](https://docs.rs/bevy/latest/bevy/prelude/struct.Transform.html). It allows a more cartoony transition, but for others this might not been what they prefer for their game theme.

It's possible to change the opacity of [`TextBundle`](https://docs.rs/bevy/latest/bevy/prelude/struct.TextBundle.html)s and background colors but that's a major hack and can't be used with [`ImageBundle`](https://docs.rs/bevy/latest/bevy/prelude/struct.ImageBundle.html)

## Additional context

https://developer.mozilla.org/en-US/docs/Web/CSS/opacity