Center the notification label on Breeze's lockscreen

and make it use the full width before wrapping

Test Plan:
Tested on the same system that reported the bug; works for all
combinations of font, font size and DPI tested.

BUG: 370281

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: davidedmundson, jensreuterberg, pritambaral, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2984