Port custom image mode to IconItem.

Summary:
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.

Reviewers: #plasma, broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D5162