Prevent MinimalScene 💥 if another scene is already loaded

[chapulina]

🦟 Bug fix

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

Summary

This is the same fix as the PRs below, applied to the MinimalScene.

• Prevent Scene3D 💥 if another scene is already loaded #347
• Prevent GzScene3D 💥 if another scene is already loaded gz-sim#1294

The MinimalScene required one extra detail. Because of all the render syncing going on, I had to explicitly shutdown the RenderWindowItem to stop all the connections from being fired.

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)
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[codecov]

Codecov Report

Merging #364 (be8668d) into ign-gui6 (dc268fe) will decrease coverage by 0.01%.
The diff coverage is 70.49%.

@@             Coverage Diff              @@
##           ign-gui6     #364      +/-   ##
============================================
- Coverage     65.40%   65.38%   -0.02%     
============================================
  Files            39       39              
  Lines          5079     5117      +38     
============================================
+ Hits           3322     3346      +24     
- Misses         1757     1771      +14     

Impacted Files	Coverage Δ
src/Application.cc	85.35% <33.33%> (-0.45%)	⬇️
src/plugins/minimal_scene/MinimalScene.cc	63.15% <72.41%> (+0.17%)	⬆️

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[ahcorde]

LGTM and tested