Merge remote-tracking branch 'origin/release-5.6' into develop

...Applications/AppClassification/app/otbImageClassifier.cxx

...plications/AppClassification/app/otbPredictRegression.cxx

...pplications/AppClassification/app/otbSampleExtraction.cxx

...ns/AppClassification/include/otbLearningApplicationBase.h

.../AppClassification/include/otbLearningApplicationBase.txx

...rning/Supervised/include/otbMachineLearningModelFactory.h

...ing/Supervised/include/otbMachineLearningModelFactory.txx

...rappers/CommandLine/src/otbWrapperCommandLineLauncher.cxx

SuperBuild/CMake/External_qt4.cmake

SuperBuild/CMakeLists.txt