Polish ToolBarApplicationHeader appearance

Summary:

• Increase the toolbar height by 4 pixels, to make it look less cramped and give items inside it more vertical padding
• Give the navigation buttons some padding

Supports T7958: Adopt Kirigami ToolBarApplicationHeader

Test Plan:
Tested with Discover.

Before:

After:

Reviewers: Kirigami, mart

Reviewed By: Kirigami, mart

Subscribers: abetts, Discover Software Store, plasma-devel, apol

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D10483