appDisplay: Revisit "Add drop shadow to icons"

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 gon, the performance implications might not be as critical (#3124

- increase blur but not much, while we can't have multi-layered shadows.

Fixes #3670
Also see #3123
11 jobs for !1642 with wip/jimmac/app-icon-shadows in 2 minutes and 24 seconds (queued for 3 seconds)
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Status Job ID Name Coverage
  Pre Review
passed check_commit_log #1132392

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passed check-merge-request #1132393

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  Prep
passed build-fedora-container #1132394

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  Review
passed eslint #1132396

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passed eslint_mr #1132397

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passed js_check #1132395

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passed no_template_check #1132399

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passed potfile_check #1132398

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  Build
passed build #1132400

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passed flatpak #1132401
flatpak

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  Test
passed test #1132402

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