Documentation generation refactoring (#10621)
* Documentation build updates:
- disable documentation by default, do not add to ALL target
- combine Doxygen and Javadoc
- optimize Doxygen html
* javadoc: fix path in build directory
* cmake: fix "Documentation" status line
Based on pull request #31 (thanks to Eric Christiansen)
Support for Java is turned off by default and can be enabled setting
BUILD_opencv_java to ON (e.g. cmake -DBUILD_opencv_java=ON ..)