Simplifies compact representation to always using IconItem, now
that IconItem supports displaying non-square image sources.
Fixes missing hover highlight effect for custom image launcher
buttons.
BUG:377652
Depends on D5160 in plasma-framework to work correctly, enabling
it to learn image source aspect ratio from the IconItem's implicit
size.
A good follow-up would be aligning Kickoff and Kicker compact
representation sizing behavior and config UI now.