Configurable icon size
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
See D4550 for a similar change.
A similar change will need to be done for Kickoff, which uses the
default CompactRepr instead.
Depends on D5593.
Reviewers: Plasma, mart
Differential Revision: https://phabricator.kde.org/D5592