Fix margins of ToolBarHeader

Summary:
I apparently accidentally changed the toolbar header's left margin to
Units.smallSpacing instead of the original Units.largeSpacing. In
addition, in some cases, the ActionToolBar would have a double right
margin due to Layout.rightMargin.

Test Plan: Titles in Kirigami Gallery now have a 10px left margin

Reviewers: Kirigami, ngraham

Reviewed By: Kirigami, ngraham

Subscribers: ngraham, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D25761