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 #1132392
check_commit_log

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passed #1132393
check-merge-request

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  Prep
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build-fedora-container

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  Review
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eslint

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passed #1132397
eslint_mr

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js_check

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passed #1132399
no_template_check

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passed #1132398
potfile_check

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  Build
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build

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flatpak
flatpak

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  Test
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test

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