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 D5592, D5594 and D4550 for similar changes.

Depends on D5593.

BUG:378443

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D5595