Improve inserting WorldControl and WorldStats from menu

[chapulina]

🦟 Bug fix

• Part of Cleanup GUI plugin menu gz-sim#1254

Summary

Currently, if WorldControl or WorldStats are inserted from the menu, they don't display anything and are therefore unusable. That's because all their elements are optional and hidden by default (which we should consider changing).

This PR works around that by displaying all widgets if no elements are passed onto the plugin. The assumption is that the user would never want to hide all elements at the same time.

With this PR, these plugins can be inserted from the menu and they work with Gazebo:

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 #349 (3b92495) into ign-gui3 (297d565) will increase coverage by 0.08%.
The diff coverage is 100.00%.

❗ Current head 3b92495 differs from pull request most recent head 85b539c. Consider uploading reports for the commit 85b539c to get more accurate results

@@             Coverage Diff              @@
##           ign-gui3     #349      +/-   ##
============================================
+ Coverage     67.71%   67.79%   +0.08%     
============================================
  Files            28       28              
  Lines          3110     3118       +8     
============================================
+ Hits           2106     2114       +8     
  Misses         1004     1004              

Impacted Files	Coverage Δ
src/plugins/world_control/WorldControl.cc	80.00% <100.00%> (+0.51%)	⬆️
src/plugins/world_stats/WorldStats.cc	97.29% <100.00%> (+0.12%)	⬆️

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[jennuine]

LGTM, left a minor comment

[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-2022-03-01-citadel-edifice-fortress/1313/1