Add functionality to add entities via the entity tree

[mjcarroll]

🎉 New feature

Summary

Put an "Insert Entity" button in the header of the entity tree.

Test it

Open gazebo with the EntityTree loaded. Click the "plus" button in the upper right and add entities.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• 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 #1101 (e2a3516) into ign-gazebo6 (827fc3f) will increase coverage by 0.10%.
The diff coverage is 53.84%.

❗ Current head e2a3516 differs from pull request most recent head 025108f. Consider uploading reports for the commit 025108f to get more accurate results

@@               Coverage Diff               @@
##           ign-gazebo6    #1101      +/-   ##
===============================================
+ Coverage        62.16%   62.27%   +0.10%     
===============================================
  Files              256      257       +1     
  Lines            20395    20440      +45     
===============================================
+ Hits             12679    12728      +49     
+ Misses            7716     7712       -4     

Impacted Files	Coverage Δ
src/gui/plugins/entity_tree/EntityTree.cc	8.06% <0.00%> (-0.95%)	⬇️
src/gui/plugins/entity_tree/EntityTree.hh	100.00% <ø> (ø)
src/gui/plugins/lights/Lights.cc	47.05% <0.00%> (+20.39%)	⬆️
src/gui/plugins/shapes/Shapes.cc	52.63% <0.00%> (+27.63%)	⬆️
src/Primitives.cc	92.45% <92.45%> (ø)

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[nkoenig]

I think @jennuine implemented the ability to drag-drop a mesh file into the GUI in order to create a model. Would it be possible to have the Mesh menu button open a file-dialog so that the user can select a mesh to add?

[mjcarroll]

I think @jennuine implemented the ability to drag-drop a mesh file into the GUI in order to create a model. Would it be possible to have the Mesh menu button open a file-dialog so that the user can select a mesh to add?

Done in ba1092b

[nkoenig]

works great