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:
{F6424115, size=full}

Lock screen:
{F6435507, size=full}

{F6435506, size=full}

{F6435519, size=full}

{F6435518, size=full}

{F6435517, size=full}

{F6435516, size=full}

{F6435515, size=full}

{F6435514, size=full}

{F6435513, size=full}

{F6435512, size=full}

{F6435511, size=full}

{F6435510, size=full}

{F6435523, size=full}

{F6435522, size=full}

{F6435521, size=full}

{F6435509, size=full}

{F6424114, size=full}

Reviewers: davidedmundson, #vdg, #plasma

Reviewed By: davidedmundson, #plasma

Subscribers: mart, rooty, plasma-devel

Tags: #plasma

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