Fix plugin filter on Windows

[peci1]

🦟 Bug fix

Summary

On Windows, the plugin combobox in GUI was always empty (both in GUI alone and in GZ GUI). This PR fixes it.

It seems to me it 'd be beneficial if this logic was extracted as helper functions to gz-plugin.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #567 (a4cef4a) into gz-gui8 (e898137) will decrease coverage by 1.96%.
The diff coverage is 100.00%.

❗ Current head a4cef4a differs from pull request most recent head 15ee40f. Consider uploading reports for the commit 15ee40f to get more accurate results

@@             Coverage Diff             @@
##           gz-gui8     #567      +/-   ##
===========================================
- Coverage    70.06%   68.11%   -1.96%     
===========================================
  Files           39       39              
  Lines         5359     5375      +16     
===========================================
- Hits          3755     3661      -94     
- Misses        1604     1714     +110     

Files	Coverage Δ
src/Application.cc	84.21% <100.00%> (+0.07%)	⬆️
src/MainWindow.cc	95.69% <100.00%> (+0.01%)	⬆️

... and 33 files with indirect coverage changes

[mjcarroll]

@osrf-jenkins retest this please