[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:

Some Updates selected:
No updates selected:

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