Use a fixed icon size for the notification popup close button

Summary:
Both the popup header size as well as units.smallMargin depend on font
size. This causes problems with the new circular timeout indicator.
Rather than trying to somehow fix the margins, this patch changes things
to use units.iconSizes.smallMedium as close button icon size instead,
stopping it from relying on font size and hopefully fixing any layout
issues.

Test Plan: Change font size to 10/11/12 pt. Icon size remains the same.

Reviewers: #plasma, #vdg, ndavis, ngraham

Reviewed By: #vdg, ndavis, ngraham

Subscribers: ndavis, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26093