support for standalone ksplash themes

Summary:
support the new standalone splashscreens of D11918
also add a GHNS button for the new splashscreen store category

FEATURE: 358839

Test Plan:
splashscreens are correctly listed, installed and removed
both ones in lnf packages and standalone are listed.

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D11919