[applets/systemtray] Simplify icon size logic
Summary: This change simplifies icon size logic - now it is controled only in one place: AbstractItem. This gives additional benefits, like icons perfectly centered.
Test Plan: Highlights should always be centered
Reviewers: Plasma: Workspaces, Plasma, ngraham, broulik, davidedmundson, apol
Reviewed By: Plasma: Workspaces, Plasma, ngraham, broulik
Subscribers: mart, davidedmundson, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D28109