bevy_ui: Ability to display an image without scaling.

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

Having images in UI that you want to be displayed exactly without any scaling. No resizing allowed.

For example, I want to create a toolbar button with an icon in it. The image assets for the button's background and the icons are intended to be fixed size and displayed as is, exactly that size.

## What solution would you like?

Currently we have an `ImageMode` enum component, which seems to have been created for this exact purpose (controlling the behavior of image scaling). It only has one variant: `KeepAspect`. It seems like the sensible/appropriate place for this new addition. We could add a new variant: `Exact`, that preserves image dimensions exactly / allows no resizing.

While we are at it, we could also add a `ImageMode::Stretch` that allows arbitrary resizing without regard for aspect ratio. Then, the `ImageMode` feature set would feel "complete" and cover the most common use cases.

## What alternative(s) have you considered?

I could use the Style properties to force the node to have exactly the size of the image, but this is boilerplatey and error-prone. It would be nice if the layout system could take care of it automatically.

Locate the bevy_ui implementation of the ImageMode component and the existing KeepAspect behavior. Review related UI layout tests or examples, then determine how exact-size and stretch behavior should be specified and tested. Done means the agreed ImageMode variants work for fixed-size and freely resized images without requiring manual style dimensions.

Written by the indexing model from the issue text.