Show release date inline with version string

Summary:
The release date is inherently tied to a specific version, so display the two together.

CCBUG: 395677

Test Plan:
{F5926563}

Also tested apps with no version and apps with a version but no release date; no regressions.

Reviewers: #discover_software_store, apol

Reviewed By: #discover_software_store, apol

Subscribers: apol, colomar, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D13656