[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:

{F6533139}

After:

{F6533140}

(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