Make "Updates Available" notification persistent but low priority

Summary:
Discover's "Updates are available" notification currently suffers from some problems:
1. With the default timeout, it disappears quickly and is likely to be missed
2. After timing out, it appears in thie history, where is is both redundant (because the Update Notifier system tray icon has become visible) and also useless (because it can't be interacted with due to https://bugs.kde.org/show_bug.cgi?id=407361 and https://bugs.kde.org/show_bug.cgi?id=407667)
3. Once updates are performed, the notification in the history sticks around uselessly because notifications can't be revoked once they're in the history (See https://bugs.kde.org/show_bug.cgi?id=409323#c3)

This patch fixes those issues by making the update persistent but low priority.

This means it must be noticed by the user and interacted with in some way before it
disappears from the screen--either by dismissing it or running the updates. But once
dismissed, it's gone forever, and doesn't clutter up your history.

BUG: 409757
BUG: 409331
FIXED-IN: 5.17.0

Test Plan:
1. Have updates available
2. Deploy patch
3. Restart plasmashell
4. See that the notification is now persistent
5. Dismiss notification
6. See that it no longer clogs up the history
7. Restart plasmashell again
8. Click on the notification's body or "Update" button
9. See that Discover opens and the notification disappears

Reviewers: apol, #discover_software_store

Reviewed By: apol, #discover_software_store

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22429