Improve KeyPublisher's usability

[chapulina]

🦟 Bug fix

Part of gazebosim/gz-sim#1254

Summary

A new user inserting the KeyPublisher plugin may get confused when they just see an empty plugin. With this PR, the plugin provides more information to the user:

This shouldn't affect use cases where the plugin is placed floating with a small size, like this.

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

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.

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[codecov]

Codecov Report

Merging #362 (467677c) into ign-gui3 (a6afb94) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           ign-gui3     #362   +/-   ##
=========================================
  Coverage     67.79%   67.79%           
=========================================
  Files            28       28           
  Lines          3118     3118           
=========================================
  Hits           2114     2114           
  Misses         1004     1004           

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[osrf-triage]

This pull request has been mentioned on Gazebo Community. There might be relevant details there:

https://community.gazebosim.org/t/new-ignition-releases-2022-04-13-fortress-edifice/1367/1