Use \n instead of <br> for the extra part of the summary

Summary:
The tooltip text only supports plaintext, so it contained a literal "<br>".
A "\n" works as a newline in both the expanded representation and the tooltip.

Test Plan:
Hovered over the icon with available security updates. Now the text
includes a newline instead of a literal "<br>".

Reviewers: lukas, jgrulich

Reviewed By: jgrulich

Subscribers: plasma-devel

Differential Revision: https://phabricator.kde.org/D12790