Make the notifications less obtrusive

Summary:
Currently, whenever the PK package cache is refreshed, a new notification
is show, even when the update count does not change. The notification
also persists after the update has installed.

To make the notifications less obtrusive:

1. Skip the "You have N new updates" message generation completely when the update count is unchanged.
2. In case the update count changes, close the old one generate a new one. Using "CloseOnTimeout" has the
3. Change the "Updates Installed" notification from "Persistent" to "CloseOnTimeout". The notification is still in the history.
4. After update installation, also remove the "You have N new updates" Popup.

Depends on D22026

Test Plan: call pkcon refresh multiple times

Reviewers: jgrulich, lukas, ngraham, fvogt

Reviewed By: ngraham, fvogt

Differential Revision: https://phabricator.kde.org/D22027