Put all systemwide add-ons in Plasma Addons category

Summary:
BUG: 390594
FIXED-IN: Plasma 5.12.2

Test Plan:
Application Addons category now only has actual app-specific addons:
{F5715679}

Plasma Addons category is now nice and full of actual Plasma (or other system-level) add-ons:
{F5715681}

Reviewers: #discover_software_store, apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D10611