[showdesktop][minimizeall] Reduce the maximum panel icon size

Summary: The current size of the showdesktop/minimizeall widget is much larger than it needs to be. This patch reduces the maximum size in panels to smallMedium (22px).

Reviewers: VDG, Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D21593