Improve update states' UI

Summary:
- Add a busy indicator when something is refreshing or checking
- Update strings to be a bit more user-friendly
- Don't change the Update page's title; status is adequately indicated by the footer text already

BUG: 402897
CCBUG: 396140
FIXED-IN: 5.16.0

Test Plan:
For example:
{F6698086, size=full}

Reviewers: apol, #vdg, #discover_software_store

Reviewed By: apol, #discover_software_store

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19825