[sddm-theme/lock screen] Tighten clock and username shadows

Summary:
This patch tightens the shadows around the clock in the Breeze SDDM theme and lock screen so as to prevent the appearance of a diffuse dark glare that appears around the clock on top of very bright backgrounds.

In addition, it tightens the username/prompt shadows to match the shadows used in desktopcontainment.

Test Plan:
Username shadows, before:

and after:

(the new shadows take after the ones in desktopcontainment)

Before (white background):

After (white background):

Before (Next):

After (Next):

Reviewers: VDG, ngraham, filipf, mart

Reviewed By: VDG, ngraham, filipf, mart

Subscribers: mart, plasma-devel

Tags: Plasma

Maniphest Tasks: T10325

Differential Revision: https://phabricator.kde.org/D19325