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:

Reviewers: apol, VDG, Discover Software Store

Reviewed By: apol, Discover Software Store

Subscribers: plasma-devel

Tags: Plasma

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