applets/kicker: use Image for non-square compactRepresentation icon

Kicker's compactRepresentation has some logic to support non-square icons, e.g. to have a wider icon (and thus clickable area) on horizontal panels. It uses Kirigami.Icon though, and that no longer supports non-square icons; it automatically scales them to be rectangular and sets fixed implicit sizes, so the code doesn't actually do anything. Kickoff had the same issue, and fixed it through an Image fallback item in 66f58480.

This change adapts the solution from kickoff to kicker.