Show about distro KCM in System Settings' System Administration category

Summary:
This is an alternative to D22728. It makes the System Information KCM appear in
System Settings' System Administration section.

Advantages over D22728:

• The information is more discoverable
• It automatically disappears if KInfoCenter is not installed or removed
• It requires no changes to SystemSettings' fragile codebase

Disadvantages:

• Since it doesn't open KInfoCenter or have a link to it, all the other information in KInfoCenter is not exposed
• If we do later want to adjust this KCM to have a button to open KInfoCenter when pushed, we'll hit https://bugs.kde.org/show_bug.cgi?id=410169 since the KCM will be already open in System Settings

FEATURE: 409811
FIXED-IN: 5.17.0

Test Plan:

Reviewers: VDG, Plasma, filipf, GB_2

Reviewed By: VDG, filipf, GB_2

Subscribers: mthw, filipf, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D22742