Introduce units.iconSizeHints.* to provide user-configurable icon size hints

Summary:
In effect this is meant to resurrect the "Panel" icon size setting from
the Icons -> Advanced KCM in System Settings, for use as a ceiling on
panel icons to avoid them taking up too much space, especially in wide
vertical panels.

Previously, the actually-unused "Desktop" size hint was added to the
iconSizes map. I believe this was in error, as the size hints are
semantically different from the size classes. This patch marks the
"desktop" iconSizes map entry as deprecated and makes it available
under iconSizeHints instead.

The soon-to-be-updated D5592 will demonstrate this API in use.

CCBUG:378443

Reviewers: Plasma, mart

Subscribers: plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D5593