Fix plugin added signal, add PluginByName

[chapulina]

🦟 Bug fix

Summary

While reviewing gazebosim/gz-sim#793 I realized that the PluginAdded signal has been emitting empty strings all along.

This PR fixes that signal, and also adds the handy PluginByName function.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• 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

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[codecov]

Codecov Report

Merging #249 (0891847) into ign-gui3 (5262650) will increase coverage by 0.01%.
The diff coverage is 87.50%.

❗ Current head 0891847 differs from pull request most recent head ce56239. Consider uploading reports for the commit ce56239 to get more accurate results

@@             Coverage Diff              @@
##           ign-gui3     #249      +/-   ##
============================================
+ Coverage     65.70%   65.71%   +0.01%     
============================================
  Files            23       23              
  Lines          2828     2835       +7     
============================================
+ Hits           1858     1863       +5     
- Misses          970      972       +2     

Impacted Files	Coverage Δ
src/Application.cc	84.59% <87.50%> (-0.31%)	⬇️

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[ahcorde]

just a minor comment