Fix imgui is not properly set when DART_USE_SYSTEM_IMGUI=ON

[jslee02]

This PR is to prevent the downstream packages need to manually add find_package(imgui), which is not ideal.

The generated dart_gui-osgComponent.cmake should contain both imgui and OpenSceneGraph.

Before:

set("dart_gui-osg_DEPENDENCIES" gui)
set("dart_gui-osg_LIBRARIES" dart-gui-osg)

set_property(DIRECTORY PROPERTY "DART_gui-osg_FOUND" TRUE)
foreach(external_dep OpenSceneGraph)
...
endforeach()

After:

set("dart_gui-osg_DEPENDENCIES" gui)
set("dart_gui-osg_LIBRARIES" dart-gui-osg)

foreach(external_dep OpenSceneGraph;imgui)
...
endforeach()

I also changed the error to a warning for building with old OSG (< 3.7.0) on macOS to allow building with the old version but with a patch for the issue.

---

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 61.55%. Comparing base (26664f5) to head (ef736b5).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1877   +/-   ##
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  Coverage   61.55%   61.55%           
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  Files         490      490           
  Lines       46013    46013           
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  Hits        28325    28325           
  Misses      17688    17688