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

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

Test Plan:
Before: {F6871329, size=full}
After: {F6871327, size=full}

Reviewers: #vdg, #plasma, mart

Reviewed By: #vdg, #plasma, mart

Subscribers: mart, ngraham, broulik, rikmills, plasma-devel

Tags: #plasma

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