refine layouts for items to be more aligned

Summary:
as per cdg feedback, remove separator lines in the list,
tweak sizes of items in the toolbar so that the hamburger icon is centered with item icons and the pixel spaces are all the same

Test Plan: tested with different QT_SCALE_FACTOR values

Reviewers: Plasma, VDG, abetts, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D6118