[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