WIP: Wip/jimmac/app icon shadows
The shadow is barely visible on dark backgrounds, but it's not wrong to be consistent with Settings, Software and GNOME Classic.
With the app icon swarm animation gone, the performance implications might not be as critical (#3124 (closed)
- increase blur but not much, while we can't have multi-layered shadows.
Fixes #3670 (closed) Also see #3123