Remove old workaround in spacing

Refine spacing and sizes of items in the sidebar header: an old workaround fixed the spacing in the toolbar to Kirigami.Units.smallSpacing/2 which didn't make much sense but was to be consistent with the old side paddings that were way too small.

Needs frameworks/kirigami!1959 (merged)

and frameworks/qqc2-desktop-style!492 (merged)