[Updates page] Improve selected/unselected explanation view

Summary:
Improve the clarity of the information that's being displayed, reduce string length
so that wordy languages don't get anything cut off, and elide properly just in case
they do.

BUG: 404603
FIXED-IN: 5.16

Test Plan:
All updates selected: {F6626922, size=full}
Some Updates selected: {F6626923, size=full}
No updates selected: {F6626925, size=full}

Reviewers: #vdg, #discover_software_store, apol

Reviewed By: #discover_software_store, apol

Subscribers: apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19192