Configurable icon size

Summary:
This patch uses the new units.iconSizeHints.panel API to limit the
icon size to what the user sets for panel icons in System Settings,
avoiding them taking up too much space, particularly in wide vertical
panels.

See D4550 for a similar change.

CCBUG:378443

A similar change will need to be done for Kickoff, which uses the
default CompactRepr instead.

Depends on D5593.

Reviewers: Plasma, mart

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D5592