Update new / removed entity GUI events

[chapulina]

🦟 Bug fix

Summary

Attempt of making the API a bit clearer after #1053 and #1138

• Made the distinction between GUI and server creation / removal
• PIMPlized events in case we want to extend them later
• Made both events consistent, including both new and removed entities
• Updated names from "added" to "new" to be consistent with other APIs
• Added unit tests

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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[adlarkin]

Changes LGTM, thanks for cleaning this up a bit! I just left a few small comments about test coverage.

[codecov]

Codecov Report

Merging #1213 (d70982e) into ign-gazebo6 (97f2e9c) will increase coverage by 0.04%.
The diff coverage is 66.66%.

@@               Coverage Diff               @@
##           ign-gazebo6    #1213      +/-   ##
===============================================
+ Coverage        61.88%   61.92%   +0.04%     
===============================================
  Files              274      275       +1     
  Lines            22441    22457      +16     
===============================================
+ Hits             13887    13907      +20     
+ Misses            8554     8550       -4     

Impacted Files	Coverage Δ
include/ignition/gazebo/gui/GuiEvents.hh	0.00% <ø> (ø)
src/gui/plugins/entity_tree/EntityTree.cc	8.06% <ø> (ø)
src/gui/plugins/scene_manager/GzSceneManager.cc	33.33% <0.00%> (-4.91%)	⬇️
src/gui/plugins/select_entities/SelectEntities.cc	10.09% <0.00%> (ø)
src/gui/GuiEvents.cc	100.00% <100.00%> (ø)

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