Distinguish external library files from DART code base

[jslee02]

Motivation

DART contains several external code files, and they are placed in the DART directories without distinction. It would be good to separate those external from the DART code base for better maintenance.

Changes

• Move external code files to separate directory (e.g., dart/external/THE_LIBRARY_NAME). The being extracted files are ImGui, lodepng, and ODE (the LCP solver part).
• Build them as separate targets and components
• Link to the targets if needed

Note

• Export each CMake component/target as a separate file #786 should be merged before this PR. Actual changes of this PR
• The targets are intentionally exposed to projects that depend on DART because they may want to use the targets. For example, osgAtlasSimbicon example uses ImGui through DART.
• One positive side effect of this change is that the code coverage measure would be a little more accurate since we don't check the external code files.
• The headers of the external code files are no longer listed in the directory headers (e.g., dart/dart.hpp and dart/dynamics/dynamics.hpp). We could add them when we find it's needed.

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