Update examples directory to make dart::gui::osg more accessible

[jslee02]

This PR tweaks the examples directory to make dart::gui::osg more accessible. Following changes are also made:

• Change example and tutorial directory names to all lower cases
• Remove InSourceBuild.cmake files in favor of dart_build[_example/_tutorial]_in_source() macros
• Format examples and tutorials code
• Add hello_world example

Resolves #1304

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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 (N/A)

[codecov]

Codecov Report

Merging #1305 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1305      +/-   ##
==========================================
+ Coverage    57.1%   57.11%   +0.01%     
==========================================
  Files         363      363              
  Lines       26954    26954              
==========================================
+ Hits        15392    15395       +3     
+ Misses      11562    11559       -3

Impacted Files	Coverage Δ
dart/dynamics/Skeleton.cpp	66.12% <0%> (+0.16%)	⬆️