[Notifications] Add padding to notifications

Summary: This patch aims to add padding to the notification widget and fully remedy the issues that arise once it is applied.

Test Plan:
Before:

After:

(More padding, and margins as even as can be)

Reviewers: VDG, Plasma, ngraham, davidedmundson

Reviewed By: VDG, Plasma, ngraham, davidedmundson

Subscribers: broulik, Codezela, abetts, filipf, davidedmundson, hein, ndavis, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D17975