[dartpy] Add GUI event handler

[jslee02]

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Before creating a pull request

• Document new methods and classes
• Format new code files using clang-format

Before merging a pull request

• Set version target by selecting a milestone on the right side
• Summarize this change in CHANGELOG.md
• Add unit test(s) for this change

[codecov]

Codecov Report

Merging #1346 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1346      +/-   ##
==========================================
- Coverage   56.89%   56.86%   -0.03%     
==========================================
  Files         366      366              
  Lines       27174    27174              
==========================================
- Hits        15461    15453       -8     
- Misses      11713    11721       +8

Impacted Files	Coverage Δ
dart/dynamics/EulerJoint.cpp	67.68% <0%> (-3.05%)	⬇️
dart/dynamics/Skeleton.cpp	65.95% <0%> (-0.17%)	⬇️