Improve sidebar header visibility

Summary:
BUG: 384638
FIXED-IN: KDE Plasma 5.12.2

System Settings' sidebar headers were implemented as orginary Kirigami list items that had `enabled: false` set, which made them turn an almost unreadable light color. This a hack; it should have used real headers. And it was the wrong hack, as evidenced by the visual breakage.

This patch changes the hack to a different one that works much better. Ideally we would use real Kirigami list headers here, but the current system settings implementation seems to make this a challenge and I'm not sure it's worth it. Willing to be proven wrong, though.

Test Plan:
Before, primary category headers, Breeze light:
{F5716438}

After, primary category headers, Breeze light:
{F5718686}

Before, sub-category headers, Breeze light:
{F5718700}

After, sub-category headers, Breeze light:
{F5718701}

Before, primary category headers, Breeze dark:
{F5716443}

After, primary category headers, Breeze dark:
{F5716442}

Before, sub-category headers, Breeze dark:
{F5718702}

After, sub-category headers, Breeze dark:
{F5718710}

Reviewers: #plasma, #kirigami, mart

Reviewed By: #plasma, #kirigami, mart

Subscribers: rkflx, plasma-devel

Tags: #plasma, #kirigami

Differential Revision: https://phabricator.kde.org/D10620