daemon: look for snap icon in icons directory as fallback

[olivercalder]

This PR is based on ~~#15003~~ #15070.

If a snap doesn't ship an icon in meta/gui/icon.*, then look for an icon for the snap in question in the snap icons install directory.

This work is tracked internally by https://warthogs.atlassian.net/browse/SNAPDENG-34437

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 90.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 78.06%. Comparing base (a272aac) to head (b7c3056). Report is 71 commits behind head on master.

Files with missing lines	Patch %	Lines
overlord/snapstate/snapstate.go	0.00%	2 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #15027      +/-   ##
==========================================
- Coverage   78.07%   78.06%   -0.01%     
==========================================
  Files        1182     1184       +2     
  Lines      157743   158172     +429     
==========================================
+ Hits       123154   123483     +329     
- Misses      26943    27014      +71     
- Partials     7646     7675      +29     

Flag	Coverage Δ
unittests	78.06% <90.00%> (-0.01%)	:arrow_down:

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[github-actions[bot]]

Wed Feb 26 21:05:39 UTC 2025 The following results are from: https://github.com/canonical/snapd/actions/runs/13548433384

Failures:

Executing:

• google:ubuntu-25.04-64:tests/main/cgroup-devices-v2
• google:ubuntu-25.04-64:tests/main/security-device-cgroups:kmsg
• google:ubuntu-25.04-64:tests/main/security-device-cgroups-required-or-optional
• google:ubuntu-25.04-64:tests/main/security-device-cgroups-strict-enforced
• google:ubuntu-25.04-64:tests/main/security-device-cgroups-helper
• google:ubuntu-25.04-64:tests/main/snap-user-service-socket-activation
• google:ubuntu-25.04-64:tests/main/security-device-cgroups-serial-port
• google:ubuntu-25.04-64:tests/main/snap-ns-forward-compat
• google:ubuntu-25.04-64:tests/main/security-device-cgroups-self-manage
• google:ubuntu-25.04-64:tests/main/security-device-cgroups:uinput

Restoring:

• google:ubuntu-25.04-64:tests/main/security-device-cgroups-strict-enforced