[Login and Lock screens] Improve UI elements' contrast a bit

Summary:
As requested in D16031, tweak the login and lock screens in only fairly non-controversial ways:

• Add a dark background behind the user avatar and a shadow behind the username
• Tweak the clock shadow to make it a tiny bit stronger
• Reduce clock shadow redundancy by moving it into the Clock item and out of the clients
• Don't use hardcoded shadow colors anymore

With the existing UI, these are very subtle changes that should barely be visible.

This diff does NOT include the following changes from D16031:

• Blur-less login screen
• New icons for the buttons
• Horizontal bar on the bottom to hold the buttons for the virtual keyboard, session chooser, and battery status

Test Plan:
Login screen:

Lock screen:

Reviewers: davidedmundson, VDG, Plasma

Reviewed By: davidedmundson, Plasma

Subscribers: mart, rooty, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D16879