[Updates page] use fancier logic for handling elided Old+New version strings

Summary:
@broulik taught me how to do this! It's more robust than the current logic, which
doesn't expand to the full form when the window becomes wide enough to show the
whole string without eliding.

Test Plan: {F7616753}

Reviewers: apol, broulik, #discover_software_store

Reviewed By: apol, #discover_software_store

Subscribers: mart, broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24713