Install required runtime for flatpakref if it's specified

Summary:
If one wants to install using flatpakref which requires a runtime we should check
if it's installed and provided by the flatpakref file to automatically install it as
well, otherwise user ends up with just application installed but not working because
of missing runtime.

Reviewers: apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D7201