bevy logo has disapeared from the `ui.rs` example

[nicopap]

It seems commit #6874 removed the image field at line 278 of the ui.rs example, and we don't see the bevy logo anymore in the example, even though there is a comment alleging the opposite.

Just adding back the line doesn't seem enough though. It's all flattened, and the label text displays with the image.

                        image: asset_server.load("branding/bevy_logo_dark_big.png").into(),

[ickshonpe]

It was broken by the alt-text implementation.
If UI nodes with a MeasureFunc (ImageBundle and TextBundle) have children then the layout algorithm ignores the MeasureFunc and derives the size of the node from the style properties like for a normal UI node.

I opened an issue about alt-text at #7928 and there was a discussion at #7894.