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[JENKINS-18154] Refactor getDoxygenGeneratedDir() to use FilePath

Using the FilePath abstraction instead of concatenating path strings
simplifies the logic substantially; it no longer matters whether the
path is relative or absolute, for example.
[JENKINS-18154] remove isDirectoryAbsolute()

The refactor of getDoxygenGeneratedDir() has made isDirectoryAbsolute()
redundant; remove it.
[JENKINS-18154] Add test for OUTPUT_DIRECTORY=.
Merge pull request #6 from kbriggs/bugfix-JENKINS-18154

[JENKINS-18154] Use correct HTML output with folderWhereYouRunDoxygen
Ugrade to 1.410
[maven-release-plugin] prepare release doxygen-0.13
[maven-release-plugin] prepare for next development iteration