As requested in D16031, tweak the login and lock screens in only fairly non-controversial ways:
- Add a dark background behind the user avatar
- Add shadows behind the username and the buttons
- 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